feat(sync): add cloud synchronization for favorites

Add a new sync service and frontend integration to allow syncing
favorites across devices using a token.

- Configure `sync-service` in docker-compose.yml on port 7482
- Add sync token input and manual sync button to SideDrawer
- Implement auto-sync logic to persist favorites to the KV store
- Add logic to merge cloud favorites with local data on initialization

nginx.conf.example

@@ -0,0 +1,27 @@
+server {
+    listen 80;
+    server_name your-domain.com;
+
+    # 前端静态资源 (Music Canvas)
+    location / {
+        proxy_pass http://127.0.0.1:7481;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    }
+
+    # 同步服务 API 代理
+    # 将 /api/kv/... 转发到 sync-service 的 7482 端口
+    # 注意：这里去掉了 /api 前缀，因为 sync-service 直接监听 /kv
+    location /api/ {
+        proxy_pass http://127.0.0.1:7482/;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        
+        # 允许跨域 (如果前端和后端不在同一个域)
+        add_header 'Access-Control-Allow-Origin' '*';
+        add_header 'Access-Control-Allow-Methods' 'GET, POST, OPTIONS';
+        add_header 'Access-Control-Allow-Headers' 'DNT,User-Agent,X-Requested-With,If-Modified-Since,Cache-Control,Content-Type,Range';
+    }
+}