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));
+  }
+}

apps/web/src/app/page.tsx

@@ -1,10 +1,45 @@
-export default function Home() {
+'use client';
+
+import { useTranslations } from 'next-intl';
+import { AppShell } from '@/components/layout/app-shell';
+
+export default function DashboardPage() {
+  const t = useTranslations('dashboard');
+
   return (
-    <main className="flex min-h-screen flex-col items-center justify-center">
-      <h1 className="text-4xl font-bold">Tessera</h1>
-      <p className="mt-4 text-lg text-gray-600">
-        Portal wird geladen...
-      </p>
-    </main>
+    <AppShell>
+      <div className="flex flex-col items-center justify-center min-h-[60vh] gap-4">
+        {/* Grid icon */}
+        <div className="rounded-lg bg-muted p-4">
+          <svg
+            xmlns="http://www.w3.org/2000/svg"
+            width="48"
+            height="48"
+            viewBox="0 0 24 24"
+            fill="none"
+            stroke="currentColor"
+            strokeWidth="1.5"
+            strokeLinecap="round"
+            strokeLinejoin="round"
+            className="text-muted-foreground"
+          >
+            <rect x="3" y="3" width="7" height="7" rx="1" />
+            <rect x="14" y="3" width="7" height="7" rx="1" />
+            <rect x="14" y="14" width="7" height="7" rx="1" />
+            <rect x="3" y="14" width="7" height="7" rx="1" />
+          </svg>
+        </div>
+
+        {/* Empty state text (D-16) */}
+        <p className="text-lg text-muted-foreground">
+          {t('empty')}
+        </p>
+
+        {/* Add widget button */}
+        <button className="mt-2 rounded-md bg-primary px-4 py-2 text-sm font-medium text-primary-foreground hover:opacity-90 transition-opacity">
+          {t('addWidget')}
+        </button>
+      </div>
+    </AppShell>
   );
 }