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/lib/db.ts

@@ -0,0 +1,57 @@
+import Dexie, { type Table } from 'dexie';
+
+export interface PendingScan {
+    id?: number;
+    imageBase64: string;
+    timestamp: number;
+    provider?: 'gemini' | 'nebius';
+}
+
+export interface PendingTasting {
+    id?: number;
+    bottle_id: string;
+    data: {
+        session_id?: string;
+        rating: number;
+        nose_notes?: string;
+        palate_notes?: string;
+        finish_notes?: string;
+        is_sample?: boolean;
+        buddy_ids?: string[];
+        tag_ids?: string[];
+    };
+    photo?: string; // Optional photo if taken during tasting
+    tasted_at: string;
+}
+
+export interface CachedTag {
+    id: string;
+    name: string;
+    category: string;
+    is_system_default: boolean;
+    popularity_score: number;
+}
+
+export interface CachedBuddy {
+    id: string;
+    name: string;
+}
+
+export class WhiskyDexie extends Dexie {
+    pending_scans!: Table<PendingScan>;
+    pending_tastings!: Table<PendingTasting>;
+    cache_tags!: Table<CachedTag>;
+    cache_buddies!: Table<CachedBuddy>;
+
+    constructor() {
+        super('WhiskyVault');
+        this.version(1).stores({
+            pending_scans: '++id, timestamp',
+            pending_tastings: '++id, bottle_id, tasted_at',
+            cache_tags: 'id, category, name',
+            cache_buddies: 'id, name'
+        });
+    }
+}
+
+export const db = new WhiskyDexie();