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/messages/de.json

@@ -3,7 +3,8 @@
     "appName": "Tessera",
     "loading": "Laden...",
     "save": "Speichern",
-    "cancel": "Abbrechen"
+    "cancel": "Abbrechen",
+    "menu": "Menu"
   },
   "header": {
     "search": "Suchen...",
@@ -18,6 +19,7 @@
     "settings": "Einstellungen",
     "collapse": "Einklappen",
     "expand": "Ausklappen",
+    "userPlaceholder": "Benutzer",
     "categories": {
       "label": "Kategorien",
       "empty": "Keine Module"