feat: implement robust offline-first sync with Dexie.js

- Migrated to Dexie.js for IndexedDB management
- Added optimistic UI for tasting notes with 'Wartet auf Sync' badge
- Implemented background caching for tags and buddies
- Unified scanning and tasting sync in a global UploadQueue

src/app/layout.tsx

@@ -9,6 +9,7 @@ import { SessionProvider } from "@/context/SessionContext";
 import ActiveSessionBanner from "@/components/ActiveSessionBanner";
 import MainContentWrapper from "@/components/MainContentWrapper";
 import AuthListener from "@/components/AuthListener";
+import SyncHandler from "@/components/SyncHandler";
 
 const inter = Inter({ subsets: ["latin"] });
 
@@ -50,6 +51,7 @@ export default function RootLayout({
                         <AuthListener />
                         <ActiveSessionBanner />
                         <MainContentWrapper>
+                            <SyncHandler />
                             <PWARegistration />
                             <OfflineIndicator />
                             <UploadQueue />