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/routes/admin/+layout.svelte

@@ -1,13 +1,14 @@
 <script lang="ts">
   import { page } from '$app/stores';
-  import { Globe, Users, DatabaseBackup, type Icon } from 'lucide-svelte';
+  import { Globe, Users, DatabaseBackup, Smartphone, type Icon } from 'lucide-svelte';
 
   let { children } = $props();
 
   const items: { href: string; label: string; icon: typeof Icon }[] = [
     { href: '/admin/domains', label: 'Domains', icon: Globe },
     { href: '/admin/profiles', label: 'Profile', icon: Users },
-    { href: '/admin/backup', label: 'Backup', icon: DatabaseBackup }
+    { href: '/admin/backup', label: 'Backup', icon: DatabaseBackup },
+    { href: '/admin/app', label: 'App', icon: Smartphone }
   ];
 </script>