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/mod.rs

@@ -4,6 +4,7 @@
 
 pub mod circuit_breaker;
 pub mod error;
+pub(crate) mod failover_switch;
 mod forwarder;
 pub mod handler_config;
 pub mod handler_context;