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/proxy/handler_context.rs

@@ -131,6 +131,8 @@ impl RequestContext {
             self.config.max_retries,
             state.status.clone(),
             state.current_providers.clone(),
+            state.failover_manager.clone(),
+            state.app_handle.clone(),
         )
     }