fix(proxy): serialize per-app provider switches to prevent state corruption

Concurrent failover switches for the same app could cause is_current,
local settings, and Live backup to point at different providers.

- Add SwitchLockManager with per-app mutexes (different apps still parallel)
- Unify scattered switch logic into ProxyService::hot_switch_provider
- Fix TOCTOU in set_current_provider via mutate_settings
- Add logical_target_changed to skip redundant UI refreshes
- Add tests for serialization and restore-waits-for-switch scenarios

src-tauri/src/proxy/mod.rs

@@ -24,6 +24,7 @@ pub mod response_processor;
 pub(crate) mod server;
 pub mod session;
 pub(crate) mod sse;
+pub(crate) mod switch_lock;
 pub mod thinking_budget_rectifier;
 pub mod thinking_optimizer;
 pub mod thinking_rectifier;