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/app/globals.css

@@ -96,3 +96,10 @@ body {
   -webkit-font-smoothing: antialiased;
   -moz-osx-font-smoothing: grayscale;
 }
+
+/* App shell main content responsive margin */
+@media (min-width: 768px) {
+  .app-shell-main {
+    margin-left: var(--current-sidebar-width, var(--sidebar-width));
+  }
+}