diff --git a/apps/api/src/auth/auth.service.spec.ts b/apps/api/src/auth/auth.service.spec.ts
index 1bafcfd..fd33767 100644
--- a/apps/api/src/auth/auth.service.spec.ts
+++ b/apps/api/src/auth/auth.service.spec.ts
@@ -49,6 +49,7 @@ interface FakeUserRow {
   mustChangePassword: boolean;
   avatarPath?: string | null;
   accentColor?: string | null;
+  dashboardBackground?: unknown;
 }
 
 interface BoundCall {
@@ -501,6 +502,8 @@ describe('AuthService.getMe', () => {
     mustChangePassword: false,
     avatarPath: 'avatars/u1.png',
     accentColor: '#3b82f6',
+    // quick-260928-ujj: gespeicherter Zusatzschluessel wird bei der Ausgabe verworfen.
+    dashboardBackground: { kind: 'preset', id: 'dunes', extra: 'weg' },
   };
 
   const ldapUserRow: FakeUserRow = {
@@ -515,6 +518,7 @@ describe('AuthService.getMe', () => {
     mustChangePassword: false,
     avatarPath: null,
     accentColor: null,
+    dashboardBackground: null,
   };
 
   beforeEach(() => {
@@ -535,6 +539,7 @@ describe('AuthService.getMe', () => {
       tenantId: 't1',
       mustChangePassword: false,
       accentColor: '#3b82f6',
+      dashboardBackground: { kind: 'preset', id: 'dunes' },
       isLocalUser: true,
       hasAvatar: true,
     });
@@ -549,6 +554,30 @@ describe('AuthService.getMe', () => {
     expect(result).toMatchObject({ isLocalUser: false, hasAvatar: false });
   });
 
+  it('quick-260928-ujj: dashboardBackground NULL (nie gewaehlt) kommt als null zurueck', async () => {
+    const result = await service.getMe('t1', 'u2');
+
+    expect(result).toHaveProperty('dashboardBackground', null);
+  });
+
+  it('quick-260928-ujj: ungueltiger gespeicherter Hintergrund kommt als null zurueck (T-ujj-01)', async () => {
+    prisma.__users.set('u1', {
+      ...prisma.__users.get('u1'),
+      dashboardBackground: { kind: 'image', imageId: '") ; background: url("x' },
+    });
+
+    const result = await service.getMe('t1', 'u1');
+
+    expect(result).toHaveProperty('dashboardBackground', null);
+  });
+
+  it('quick-260928-ujj: dashboardBackground steht im select neben accentColor', async () => {
+    await service.getMe('t1', 'u1');
+
+    const call = prisma.__boundCallLog.find((c: any) => c.method === 'findUnique');
+    expect(call.args.select).toMatchObject({ accentColor: true, dashboardBackground: true });
+  });
+
   it('FREMDER Mandant (Klient unter t2, Zeile unter t1): liefert null, kein Fehler', async () => {
     const result = await service.getMe('t2', 'u1');
 
diff --git a/apps/api/src/user/user.controller.spec.ts b/apps/api/src/user/user.controller.spec.ts
index fd2ab74..0ca46e3 100644
--- a/apps/api/src/user/user.controller.spec.ts
+++ b/apps/api/src/user/user.controller.spec.ts
@@ -580,4 +580,87 @@ describe('UserController', () => {
       });
     });
   });
+
+  /**
+   * quick-260928-ujj: Dashboard-Hintergrund pro Benutzer in der Datenbank.
+   * Der Selbstbedienungsweg schreibt ausschliesslich die Zeile des
+   * angemeldeten Benutzers ueber den an dessen Mandanten gebundenen Klienten
+   * (T-ujj-02) und nimmt nur die erlaubten Formen an (T-ujj-01): gespeichert
+   * wird immer ein frisch aufgebautes Objekt ohne Zusatzschluessel.
+   */
+  describe('Dashboard-Hintergrund (quick-260928-ujj)', () => {
+    const me = { role: Role.USER, tenantId: 't1', id: 'u-me', username: 'me', mustChangePassword: false };
+    const IMAGE_ID = '3f2b8c1e-9a4d-4e7f-8b21-0c5d6e7f8a9b';
+
+    function seedMe(tenantId = 't1', id = 'u-me') {
+      prisma.__seedUser({ id, username: id, tenantId, dashboardBackground: null });
+    }
+
+    async function storedFor(tenantId: string, id: string) {
+      const row = await prisma.__makeBoundClient(tenantId).user.findUnique({ where: { id } });
+      return row.dashboardBackground;
+    }
+
+    it.each([
+      [{ kind: 'none' }, { kind: 'none' }],
+      [{ kind: 'preset', id: 'dunes' }, { kind: 'preset', id: 'dunes' }],
+      [{ kind: 'preset', id: 'mosaic', extra: 'weg' }, { kind: 'preset', id: 'mosaic' }],
+      [{ kind: 'image', imageId: IMAGE_ID }, { kind: 'image', imageId: IMAGE_ID }],
+      [{ kind: 'image', imageId: IMAGE_ID.toUpperCase(), id: 'mist' }, { kind: 'image', imageId: IMAGE_ID.toUpperCase() }],
+      [{ kind: 'none', id: 'dunes', imageId: IMAGE_ID }, { kind: 'none' }],
+    ])('%j wird normalisiert als %j gespeichert, gebunden an den eigenen Mandanten', async (input, expected) => {
+      seedMe();
+
+      const result = await controller.updateDashboardBackground({ background: input }, me);
+
+      expect(result).toEqual({ success: true, dashboardBackground: expected });
+      expect(await storedFor('t1', 'u-me')).toEqual(expected);
+      expectBoundCall(prisma, 't1', 'user', 'update');
+      expect(prisma.__boundCallLog.filter((c: any) => c.method === 'update').every((c: any) => c.tenantId === 't1')).toBe(true);
+    });
+
+    it.each([
+      [{ kind: 'preset', id: 'regenbogen' }],
+      [{ kind: 'preset' }],
+      [{ kind: 'preset', id: 42 }],
+      [{ kind: 'gradient', id: 'mist' }],
+      [{ kind: 'image', imageId: 'img-1' }],
+      [{ kind: 'image', imageId: '") ; background: url("https://boese.invalid/x' }],
+      [{ kind: 'image', imageId: `${IMAGE_ID})` }],
+      [{ kind: 'image', imageId: `${IMAGE_ID}0` }],
+      [{ kind: 'image', imageId: '' }],
+      [{ kind: 'image' }],
+      [{}],
+      [null],
+      [undefined],
+      ['none'],
+      [42],
+      [[{ kind: 'none' }]],
+    ])('ungueltige Wahl %j → BadRequestException, nichts geschrieben', async (background) => {
+      seedMe();
+
+      await expect(controller.updateDashboardBackground({ background }, me)).rejects.toBeInstanceOf(BadRequestException);
+      expect(prisma.__boundCallLog.some((c: any) => c.method === 'update')).toBe(false);
+      expect(await storedFor('t1', 'u-me')).toBeNull();
+    });
+
+    it('fehlender Rumpf → BadRequestException', async () => {
+      seedMe();
+
+      await expect(
+        controller.updateDashboardBackground(undefined as unknown as { background: unknown }, me),
+      ).rejects.toBeInstanceOf(BadRequestException);
+      expect(prisma.__boundCallLog.some((c: any) => c.method === 'update')).toBe(false);
+    });
+
+    it('zwei Benutzer in zwei Mandanten: nur die Zeile des Anfragenden aendert sich', async () => {
+      seedMe('t1', 'u-me');
+      seedMe('t2', 'u-other');
+
+      await controller.updateDashboardBackground({ background: { kind: 'preset', id: 'mist' } }, me);
+
+      expect(await storedFor('t1', 'u-me')).toEqual({ kind: 'preset', id: 'mist' });
+      expect(await storedFor('t2', 'u-other')).toBeNull();
+    });
+  });
 });
diff --git a/apps/web/src/components/dashboard/dashboard-background.test.tsx b/apps/web/src/components/dashboard/dashboard-background.test.tsx
new file mode 100644
index 0000000..6bf2943
--- /dev/null
+++ b/apps/web/src/components/dashboard/dashboard-background.test.tsx
@@ -0,0 +1,129 @@
+import { act, renderHook, waitFor } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { type AuthUser, useAuthStore } from '@/lib/stores/auth-store';
+
+// quick-260928-ujj: der Dashboard-Hintergrund steht pro Benutzer in der
+// Datenbank. Der Hook liest ihn aus dem Auth-Store (Sitzungsantwort) und
+// speichert ueber die Server-Aktion; eine alte localStorage-Wahl wird
+// einmalig uebernommen.
+
+const updateDashboardBackgroundAction = vi.fn();
+
+vi.mock('@/lib/auth-actions', () => ({
+  updateDashboardBackgroundAction: (...args: unknown[]) => updateDashboardBackgroundAction(...args),
+}));
+
+const LEGACY_KEY = 'tessera.dashboardBackground.u1';
+
+function makeUser(overrides: Partial<AuthUser> = {}): AuthUser {
+  return {
+    id: 'u1',
+    username: 'schalli',
+    displayName: 'Schalli',
+    role: 'USER',
+    tenantId: 't1',
+    accentColor: null,
+    dashboardBackground: null,
+    ...overrides,
+  };
+}
+
+async function renderBackgroundHook() {
+  const { useDashboardBackground } = await import('./dashboard-background');
+  return renderHook(() => useDashboardBackground());
+}
+
+beforeEach(() => {
+  updateDashboardBackgroundAction.mockReset();
+  updateDashboardBackgroundAction.mockResolvedValue({ success: true });
+  useAuthStore.setState({ user: null });
+});
+
+afterEach(() => {
+  window.localStorage.clear();
+});
+
+describe('useDashboardBackground (quick-260928-ujj)', () => {
+  it('ohne angemeldeten Benutzer: kein Hintergrund, kein Speichern', async () => {
+    const { result } = await renderBackgroundHook();
+
+    expect(result.current.background).toEqual({ kind: 'none' });
+    expect(updateDashboardBackgroundAction).not.toHaveBeenCalled();
+  });
+
+  it('liest die Wahl aus dem Auth-Store; null ergibt „kein Hintergrund“', async () => {
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: { kind: 'preset', id: 'dunes' } }) });
+    const { result } = await renderBackgroundHook();
+    expect(result.current.background).toEqual({ kind: 'preset', id: 'dunes' });
+
+    act(() => useAuthStore.setState({ user: makeUser({ dashboardBackground: null }) }));
+    expect(result.current.background).toEqual({ kind: 'none' });
+  });
+
+  it('choose() setzt den Store sofort und speichert ueber die Server-Aktion', async () => {
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: { kind: 'none' } }) });
+    const { result } = await renderBackgroundHook();
+
+    act(() => {
+      result.current.choose({ kind: 'preset', id: 'mosaic' });
+    });
+
+    expect(useAuthStore.getState().user?.dashboardBackground).toEqual({ kind: 'preset', id: 'mosaic' });
+    expect(result.current.background).toEqual({ kind: 'preset', id: 'mosaic' });
+    expect(updateDashboardBackgroundAction).toHaveBeenCalledWith({ kind: 'preset', id: 'mosaic' });
+  });
+
+  it.each([
+    ['Fehlerantwort', () => updateDashboardBackgroundAction.mockResolvedValue({ success: false, error: 'saveError' })],
+    ['Ausnahme', () => updateDashboardBackgroundAction.mockRejectedValue(new Error('offline'))],
+  ])('Speichern schlaegt fehl (%s): der vorige Wert wird zurueckgesetzt', async (_label, arrange) => {
+    arrange();
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: { kind: 'preset', id: 'dunes' } }) });
+    const { result } = await renderBackgroundHook();
+
+    act(() => {
+      result.current.choose({ kind: 'preset', id: 'mist' });
+    });
+
+    await waitFor(() => expect(result.current.background).toEqual({ kind: 'preset', id: 'dunes' }));
+    expect(useAuthStore.getState().user?.dashboardBackground).toEqual({ kind: 'preset', id: 'dunes' });
+  });
+
+  it('Server-Wert null und alte localStorage-Wahl: wird genau einmal gespeichert, Schluessel entfernt', async () => {
+    window.localStorage.setItem(LEGACY_KEY, JSON.stringify({ kind: 'preset', id: 'pebble' }));
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: null }) });
+
+    const { result, rerender } = await renderBackgroundHook();
+
+    await waitFor(() => expect(updateDashboardBackgroundAction).toHaveBeenCalledTimes(1));
+    expect(updateDashboardBackgroundAction).toHaveBeenCalledWith({ kind: 'preset', id: 'pebble' });
+    expect(result.current.background).toEqual({ kind: 'preset', id: 'pebble' });
+    expect(window.localStorage.getItem(LEGACY_KEY)).toBeNull();
+
+    // erneutes Rendern und ein neues Benutzerobjekt desselben Benutzers loesen nichts mehr aus
+    rerender();
+    act(() => useAuthStore.setState({ user: makeUser({ dashboardBackground: null }) }));
+    rerender();
+    expect(updateDashboardBackgroundAction).toHaveBeenCalledTimes(1);
+  });
+
+  it('Server-Wert gesetzt: keine Uebernahme, der alte Schluessel wird nur aufgeraeumt', async () => {
+    window.localStorage.setItem(LEGACY_KEY, JSON.stringify({ kind: 'preset', id: 'pebble' }));
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: { kind: 'none' } }) });
+
+    const { result } = await renderBackgroundHook();
+
+    await waitFor(() => expect(window.localStorage.getItem(LEGACY_KEY)).toBeNull());
+    expect(updateDashboardBackgroundAction).not.toHaveBeenCalled();
+    expect(result.current.background).toEqual({ kind: 'none' });
+  });
+
+  it('Server-Wert null ohne alte Wahl: nichts wird gespeichert', async () => {
+    useAuthStore.setState({ user: makeUser({ dashboardBackground: null }) });
+
+    const { result } = await renderBackgroundHook();
+
+    expect(result.current.background).toEqual({ kind: 'none' });
+    expect(updateDashboardBackgroundAction).not.toHaveBeenCalled();
+  });
+});
diff --git a/apps/web/src/components/layout/header.test.tsx b/apps/web/src/components/layout/header.test.tsx
index e99e1e3..5b38aca 100644
--- a/apps/web/src/components/layout/header.test.tsx
+++ b/apps/web/src/components/layout/header.test.tsx
@@ -103,6 +103,7 @@ describe('Header — Sitzungswaechter (quick-260917-gyd)', () => {
         tenantId: 't1',
         hasAvatar: false,
         accentColor: null,
+        dashboardBackground: { kind: 'preset', id: 'dunes' },
       },
     });
 
@@ -111,6 +112,8 @@ describe('Header — Sitzungswaechter (quick-260917-gyd)', () => {
     await waitFor(() =>
       expect(useAuthStore.getState().user?.username).toBe('schalli'),
     );
+    // quick-260928-ujj: der Hintergrund kommt mit derselben Sitzungsantwort wie accentColor.
+    expect(useAuthStore.getState().user?.dashboardBackground).toEqual({ kind: 'preset', id: 'dunes' });
     expect((window as any).location.href).toBe('');
   });
 
diff --git a/apps/web/src/lib/dashboard-background.test.ts b/apps/web/src/lib/dashboard-background.test.ts
index ddbf377..77b24a3 100644
--- a/apps/web/src/lib/dashboard-background.test.ts
+++ b/apps/web/src/lib/dashboard-background.test.ts
@@ -1,48 +1,72 @@
+import { DASHBOARD_BACKGROUND_PRESET_IDS } from '@tessera/shared';
 import { afterEach, describe, expect, it, vi } from 'vitest';
 import {
   BACKGROUND_PRESETS,
   DARK_FALLBACK_ID,
-  loadDashboardBackground,
   presetBackground,
-  saveDashboardBackground,
+  takeLegacyDashboardBackground,
 } from './dashboard-background';
 
+const IMAGE_ID = '3f2b8c1e-9a4d-4e7f-8b21-0c5d6e7f8a9b';
+
 afterEach(() => {
   window.localStorage.clear();
   vi.restoreAllMocks();
 });
 
-describe('dashboard-background (Design „Mosaik“, Prototyp)', () => {
-  it('ohne gespeicherte Wahl: kein Hintergrund', () => {
-    expect(loadDashboardBackground('u1')).toEqual({ kind: 'none' });
-  });
-
-  it('speichert je Benutzer getrennt', () => {
-    saveDashboardBackground('u1', { kind: 'preset', id: 'bloom' });
-    saveDashboardBackground('u2', { kind: 'image', imageId: 'img-1' });
-    expect(loadDashboardBackground('u1')).toEqual({ kind: 'preset', id: 'bloom' });
-    expect(loadDashboardBackground('u2')).toEqual({ kind: 'image', imageId: 'img-1' });
-  });
-
-  it('verwirft kaputte oder unbekannte Werte', () => {
-    window.localStorage.setItem('tessera.dashboardBackground.u1', '{kaputt');
-    expect(loadDashboardBackground('u1')).toEqual({ kind: 'none' });
-    window.localStorage.setItem(
-      'tessera.dashboardBackground.u1',
-      JSON.stringify({ kind: 'preset', id: 'regenbogen' }),
-    );
-    expect(loadDashboardBackground('u1')).toEqual({ kind: 'none' });
-  });
-
-  it('gesperrter Speicher wirft nicht', () => {
-    vi.spyOn(Storage.prototype, 'getItem').mockImplementation(() => {
-      throw new Error('blocked');
+describe('dashboard-background (Design „Mosaik“)', () => {
+  describe('takeLegacyDashboardBackground (quick-260928-ujj)', () => {
+    it('ohne alte Wahl: null', () => {
+      expect(takeLegacyDashboardBackground('u1')).toBeNull();
     });
-    vi.spyOn(Storage.prototype, 'setItem').mockImplementation(() => {
-      throw new Error('blocked');
+
+    it('liefert eine gueltige alte Wahl je Benutzer und entfernt den Schluessel', () => {
+      window.localStorage.setItem(
+        'tessera.dashboardBackground.u1',
+        JSON.stringify({ kind: 'preset', id: 'bloom' }),
+      );
+      window.localStorage.setItem(
+        'tessera.dashboardBackground.u2',
+        JSON.stringify({ kind: 'image', imageId: IMAGE_ID }),
+      );
+
+      expect(takeLegacyDashboardBackground('u1')).toEqual({ kind: 'preset', id: 'bloom' });
+      expect(window.localStorage.getItem('tessera.dashboardBackground.u1')).toBeNull();
+      // der Schluessel des anderen Benutzers bleibt, bis dieser sich anmeldet
+      expect(window.localStorage.getItem('tessera.dashboardBackground.u2')).not.toBeNull();
+      expect(takeLegacyDashboardBackground('u2')).toEqual({ kind: 'image', imageId: IMAGE_ID });
+      // zweiter Aufruf: nichts mehr da
+      expect(takeLegacyDashboardBackground('u1')).toBeNull();
     });
-    expect(() => saveDashboardBackground('u1', { kind: 'preset', id: 'mist' })).not.toThrow();
-    expect(loadDashboardBackground('u1')).toEqual({ kind: 'none' });
+
+    it.each([
+      ['{kaputt'],
+      [JSON.stringify({ kind: 'preset', id: 'regenbogen' })],
+      // Prototyp-Werte mit beliebiger Bildkennung werden nicht uebernommen
+      [JSON.stringify({ kind: 'image', imageId: 'img-1' })],
+      [JSON.stringify(null)],
+    ])('ungueltiger Wert %j: null, Schluessel entfernt', (raw) => {
+      window.localStorage.setItem('tessera.dashboardBackground.u1', raw);
+
+      expect(takeLegacyDashboardBackground('u1')).toBeNull();
+      expect(window.localStorage.getItem('tessera.dashboardBackground.u1')).toBeNull();
+    });
+
+    it('gesperrter Speicher: null ohne Ausnahme', () => {
+      vi.spyOn(Storage.prototype, 'getItem').mockImplementation(() => {
+        throw new Error('blocked');
+      });
+      vi.spyOn(Storage.prototype, 'removeItem').mockImplementation(() => {
+        throw new Error('blocked');
+      });
+
+      expect(() => takeLegacyDashboardBackground('u1')).not.toThrow();
+      expect(takeLegacyDashboardBackground('u1')).toBeNull();
+    });
+  });
+
+  it('die eingebauten Hintergruende sind deckungsgleich mit der Pruefregel in @tessera/shared', () => {
+    expect(BACKGROUND_PRESETS.map((p) => p.id).sort()).toEqual([...DASHBOARD_BACKGROUND_PRESET_IDS].sort());
   });
 
   it('jeder eingebaute Hintergrund liefert hell und dunkel ein Bild (dunkel notfalls den Ersatz)', () => {