feat(pwa): Admin-Tab "App" mit Install + Sync + Cache-Reset

Neuer vierter Admin-Tab (Smartphone-Icon) mit drei Karten:
1. Installieren — fängt beforeinstallprompt (Android), zeigt
   iOS-Teilen-Hinweis, sonst Info "nicht verfügbar".
2. Offline-Synchronisation — Status + "Jetzt synchronisieren"-
   Button, disabled wenn offline.
3. Cache — "Offline-Cache leeren" löscht alle kochwas-*-Caches
   via caches.keys() + delete.

install-prompt.svelte.ts hält das deferred-Event und die Plattform
(android/ios/other) per UA-Detection.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/client/install-prompt.svelte.ts

@@ -0,0 +1,44 @@
+// Captures the beforeinstallprompt event (Android Chrome) and holds it for
+// manual triggering by the user. On iOS Safari this event does not exist —
+// we detect the browser via UserAgent and show an info hint instead.
+class InstallPromptStore {
+  available = $state(false);
+  platform = $state<'android' | 'ios' | 'other'>('other');
+  private deferred: BeforeInstallPromptEvent | null = null;
+
+  init(): void {
+    if (typeof window === 'undefined') return;
+    this.platform = detectPlatform();
+    window.addEventListener('beforeinstallprompt', (e) => {
+      e.preventDefault();
+      this.deferred = e as BeforeInstallPromptEvent;
+      this.available = true;
+    });
+    window.addEventListener('appinstalled', () => {
+      this.deferred = null;
+      this.available = false;
+    });
+  }
+
+  async prompt(): Promise<void> {
+    if (!this.deferred) return;
+    await this.deferred.prompt();
+    this.deferred = null;
+    this.available = false;
+  }
+}
+
+function detectPlatform(): 'android' | 'ios' | 'other' {
+  const ua = navigator.userAgent;
+  if (/iPhone|iPad|iPod/i.test(ua)) return 'ios';
+  if (/Android/i.test(ua)) return 'android';
+  return 'other';
+}
+
+// Minimal type for the Chrome-specific event
+type BeforeInstallPromptEvent = Event & {
+  prompt: () => Promise<void>;
+  userChoice: Promise<{ outcome: 'accepted' | 'dismissed' }>;
+};
+
+export const installPrompt = new InstallPromptStore();