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/components/layout/app-shell.tsx

@@ -0,0 +1,34 @@
+'use client';
+
+import { useEffect, useState } from 'react';
+import { useSidebarStore } from '@/lib/stores/sidebar-store';
+import { Header } from '@/components/layout/header';
+import { Sidebar } from '@/components/layout/sidebar';
+
+export function AppShell({ children }: { children: React.ReactNode }) {
+  const isCollapsed = useSidebarStore((s) => s.isCollapsed);
+  const [mounted, setMounted] = useState(false);
+
+  useEffect(() => {
+    setMounted(true);
+  }, []);
+
+  // Desktop: sidebar pushes content via margin-left
+  // Mobile: sidebar overlays, no margin needed (handled by app-shell-main class in globals.css)
+  const sidebarWidth = isCollapsed
+    ? 'var(--sidebar-width-collapsed)'
+    : 'var(--sidebar-width)';
+
+  return (
+    <div className="min-h-screen bg-background">
+      <Header />
+      <Sidebar />
+      <main
+        className="app-shell-main min-h-[calc(100vh-var(--header-height))] transition-all duration-200 p-6"
+        style={mounted ? { '--current-sidebar-width': sidebarWidth } as React.CSSProperties : undefined}
+      >
+        {children}
+      </main>
+    </div>
+  );
+}