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/locale-switcher.tsx

@@ -0,0 +1,33 @@
+'use client';
+
+import { useRouter } from 'next/navigation';
+import { useLocale, useTranslations } from 'next-intl';
+import { useTransition } from 'react';
+
+export function LocaleSwitcher() {
+  const router = useRouter();
+  const locale = useLocale();
+  const t = useTranslations('locale');
+  const [isPending, startTransition] = useTransition();
+
+  function switchLocale() {
+    const newLocale = locale === 'de' ? 'en' : 'de';
+    document.cookie = `NEXT_LOCALE=${newLocale};path=/;max-age=31536000`;
+    startTransition(() => {
+      router.refresh();
+    });
+  }
+
+  const currentLabel = locale === 'de' ? t('de') : t('en');
+
+  return (
+    <button
+      onClick={switchLocale}
+      disabled={isPending}
+      className="inline-flex items-center justify-center rounded-md px-2.5 py-1.5 text-sm font-medium text-muted-foreground hover:bg-muted hover:text-foreground transition-colors disabled:opacity-50"
+      aria-label={currentLabel}
+    >
+      {locale === 'de' ? 'EN' : 'DE'}
+    </button>
+  );
+}