feat(proxy): sync UI when failover succeeds

Add FailoverSwitchManager to handle provider switching after successful
failover. This ensures the UI reflects the actual provider in use:

- Create failover_switch.rs with deduplication and async switching logic
- Pass AppHandle through ProxyService -> ProxyServer -> RequestForwarder
- Update is_current in database when failover succeeds
- Emit provider-switched event for frontend refresh
- Update tray menu and live backup synchronously

The switching runs asynchronously via tokio::spawn to avoid blocking
API responses while still providing immediate UI feedback.

src-tauri/src/lib.rs

@@ -332,6 +332,11 @@ pub fn run() {
 
             let app_state = AppState::new(db);
 
+            // 设置 AppHandle 用于代理故障转移时的 UI 更新
+            app_state
+                .proxy_service
+                .set_app_handle(app.handle().clone());
+
             // ============================================================
             // 按表独立判断的导入逻辑（各类数据独立检查，互不影响）
             // ============================================================