fix(failover): switch to P1 immediately when enabling auto failover

Previously, enabling auto failover kept using the current provider until
the first failure, causing inconsistency when the current provider was
not in the failover queue. When stopping proxy, the restored config
would not match user expectations.

New behavior:
- Enable auto failover = immediately switch to queue P1
- Subsequent routing follows queue order (P1→P2→...)
- Auto-add current provider to queue if queue is empty

Changes:
- Add switch_proxy_target() for hot-switching during proxy mode
- Update provider_router to use queue order when failover enabled
- Sync tray menu Auto click with the same logic
- Update UI tooltips to reflect new semantics
- Add tests for queue-only routing scenario

src-tauri/src/services/proxy.rs

@@ -1530,7 +1530,10 @@ impl ProxyService {
         app_type: &str,
         provider_id: &str,
     ) -> Result<(), String> {
-        // 更新数据库中的 is_current 标记
+        // 代理模式切换供应商（热切换）：
+        // - 更新 SSOT（数据库 is_current）
+        // - 同步本地 settings（设备级 current_provider_*）
+        // - 若该应用正处于接管模式，则同步更新 Live 备份（用于停止代理时恢复）
         let app_type_enum =
             AppType::from_str(app_type).map_err(|_| format!("无效的应用类型: {app_type}"))?;
 
@@ -1538,6 +1541,35 @@ impl ProxyService {
             .set_current_provider(app_type_enum.as_str(), provider_id)
             .map_err(|e| format!("更新当前供应商失败: {e}"))?;
 
+        // 同步本地 settings（设备级优先）
+        crate::settings::set_current_provider(&app_type_enum, Some(provider_id))
+            .map_err(|e| format!("更新本地当前供应商失败: {e}"))?;
+
+        // 仅在确实处于接管状态时才更新 Live 备份，避免无接管时误写覆盖 Live
+        let has_backup = self
+            .db
+            .get_live_backup(app_type_enum.as_str())
+            .await
+            .ok()
+            .flatten()
+            .is_some();
+        let live_taken_over = self.detect_takeover_in_live_config_for_app(&app_type_enum);
+
+        if let Ok(Some(provider)) = self.db.get_provider_by_id(provider_id, app_type) {
+            // 同步更新 Live 备份（用于 stop_with_restore 恢复）
+            if has_backup || live_taken_over {
+                self.update_live_backup_from_provider(app_type, &provider)
+                    .await?;
+            }
+
+            // 同步更新 ProxyStatus.active_targets（用于 UI 立即反映切换目标）
+            if let Some(server) = self.server.read().await.as_ref() {
+                server
+                    .set_active_target(app_type_enum.as_str(), &provider.id, &provider.name)
+                    .await;
+            }
+        }
+
         log::info!("代理模式：已切换 {app_type} 的目标供应商为 {provider_id}");
         Ok(())
     }
@@ -2058,4 +2090,66 @@ model = "gpt-5.1-codex"
             "should not add ANTHROPIC_AUTH_TOKEN when absent"
         );
     }
+
+    #[tokio::test]
+    #[serial]
+    async fn switch_proxy_target_updates_live_backup_when_taken_over() {
+        let _home = TempHome::new();
+        crate::settings::reload_settings().expect("reload settings");
+
+        let db = Arc::new(Database::memory().expect("init db"));
+        let service = ProxyService::new(db.clone());
+
+        let provider_a = Provider::with_id(
+            "a".to_string(),
+            "A".to_string(),
+            json!({
+                "env": {
+                    "ANTHROPIC_API_KEY": "a-key"
+                }
+            }),
+            None,
+        );
+        let provider_b = Provider::with_id(
+            "b".to_string(),
+            "B".to_string(),
+            json!({
+                "env": {
+                    "ANTHROPIC_API_KEY": "b-key"
+                }
+            }),
+            None,
+        );
+        db.save_provider("claude", &provider_a)
+            .expect("save provider a");
+        db.save_provider("claude", &provider_b)
+            .expect("save provider b");
+        db.set_current_provider("claude", "a")
+            .expect("set current provider");
+
+        // 模拟“已接管”状态：存在 Live 备份（内容不重要，会被热切换更新）
+        db.save_live_backup("claude", "{\"env\":{}}")
+            .await
+            .expect("seed live backup");
+
+        service
+            .switch_proxy_target("claude", "b")
+            .await
+            .expect("switch proxy target");
+
+        // 断言：本地 settings 的 current provider 已同步
+        assert_eq!(
+            crate::settings::get_current_provider(&AppType::Claude).as_deref(),
+            Some("b")
+        );
+
+        // 断言：Live 备份已更新为目标供应商配置（用于 stop_with_restore 恢复）
+        let backup = db
+            .get_live_backup("claude")
+            .await
+            .expect("get live backup")
+            .expect("backup exists");
+        let expected = serde_json::to_string(&provider_b.settings_config).expect("serialize");
+        assert_eq!(backup.original_config, expected);
+    }
 }