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/hooks/useCacheSync.ts

@@ -0,0 +1,44 @@
+import { useEffect } from 'react';
+import { createClientComponentClient } from '@supabase/auth-helpers-nextjs';
+import { db } from '@/lib/db';
+import { getAllSystemTags } from '@/services/tags';
+
+export function useCacheSync() {
+    const supabase = createClientComponentClient();
+
+    useEffect(() => {
+        const syncCache = async () => {
+            if (!navigator.onLine) return;
+
+            try {
+                // 1. Sync Tags
+                const tags = await getAllSystemTags();
+                if (tags.length > 0) {
+                    await db.cache_tags.bulkPut(tags.map(t => ({
+                        id: t.id,
+                        name: t.name,
+                        category: t.category,
+                        is_system_default: t.is_system_default,
+                        popularity_score: t.popularity_score || 0
+                    })));
+                }
+
+                // 2. Sync Buddies
+                const { data: buddies } = await supabase
+                    .from('buddies')
+                    .select('id, name')
+                    .order('name');
+
+                if (buddies && buddies.length > 0) {
+                    await db.cache_buddies.bulkPut(buddies);
+                }
+            } catch (err) {
+                console.error('Failed to sync offline cache:', err);
+            }
+        };
+
+        syncCache();
+        window.addEventListener('online', syncCache);
+        return () => window.removeEventListener('online', syncCache);
+    }, [supabase]);
+}