feat(failover): add auto-failover master switch with proxy integration (#427)

* feat(failover): add auto-failover master switch with proxy integration

- Add persistent auto_failover_enabled setting in database
- Add get/set_auto_failover_enabled commands
- Provider router respects master switch state
- Proxy shutdown automatically disables failover
- Enabling failover auto-starts proxy server
- Optimistic updates for failover queue toggle

* feat(proxy): persist proxy takeover state across app restarts

- Add proxy_takeover_{app_type} settings for per-app state tracking
- Restore proxy takeover state automatically on app startup
- Preserve state on normal exit, clear on manual stop
- Add stop_with_restore_keep_state method for graceful shutdown

* fix(proxy): set takeover state for all apps in start_with_takeover

src/lib/api/failover.ts

@@ -104,4 +104,14 @@ export const failoverApi = {
       enabled,
     });
   },
+
+  // 获取自动故障转移总开关状态
+  async getAutoFailoverEnabled(): Promise<boolean> {
+    return invoke("get_auto_failover_enabled");
+  },
+
+  // 设置自动故障转移总开关状态
+  async setAutoFailoverEnabled(enabled: boolean): Promise<void> {
+    return invoke("set_auto_failover_enabled", { enabled });
+  },
 };