feat(01-02): portal layout with header, sidebar, theme toggle, locale switcher

- Zustand sidebar store with persist middleware (collapse/expand, mobile open)
- Sticky header with logo, breadcrumb, theme toggle, locale switcher, user avatar
- Collapsible sidebar with Dashboard/Marketplace nav, accordion categories, footer
- Mobile responsive: hamburger menu with overlay sidebar on small screens
- Theme toggle cycling light/dark/system with mounted guard
- Locale switcher setting NEXT_LOCALE cookie with router.refresh
- Empty dashboard state with grid icon, "Keine Widgets aktiv" text, add button
- AppShell composing header + sidebar + responsive main content area
- All user-visible strings via useTranslations (UI-03 compliance)

apps/web/src/lib/stores/sidebar-store.ts

@@ -0,0 +1,21 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+interface SidebarState {
+  isCollapsed: boolean;
+  isMobileOpen: boolean;
+  toggle: () => void;
+  setMobileOpen: (open: boolean) => void;
+}
+
+export const useSidebarStore = create<SidebarState>()(
+  persist(
+    (set) => ({
+      isCollapsed: false,
+      isMobileOpen: false,
+      toggle: () => set((state) => ({ isCollapsed: !state.isCollapsed })),
+      setMobileOpen: (open) => set({ isMobileOpen: open }),
+    }),
+    { name: 'tessera-sidebar' }
+  )
+);