feat(260928-ujj): Dashboard-Hintergrund pro Benutzer in der Datenbank

- Spalte User.dashboardBackground (JSONB) samt Migration
- PATCH /users/me/dashboard-background, geprueft mit parseDashboardBackground aus @tessera/shared (Allowlist, UUID-Bildkennung)
- getMe liefert dashboardBackground normalisiert neben accentColor
- Web liest die Wahl aus dem Auth-Store, speichert ueber die Server-Aktion, alte localStorage-Wahl wird einmalig uebernommen
- Hinweistext: gilt auf jedem Geraet

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -127,7 +127,7 @@
     "background": {
       "button": "Background",
       "title": "Dashboard background",
-      "hint": "Applies only to you and only in this browser.",
+      "hint": "Applies only to you – on every device you sign in on.",
       "builtIn": "Built-in",
       "none": "None",
       "ownImages": "Your images",