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/failover_switch.rs

@@ -0,0 +1,148 @@
+//! 故障转移切换模块
+//!
+//! 处理故障转移成功后的供应商切换逻辑，包括：
+//! - 去重控制（避免多个请求同时触发）
+//! - 数据库更新
+//! - 托盘菜单更新
+//! - 前端事件发射
+//! - Live 备份更新
+
+use crate::database::Database;
+use crate::error::AppError;
+use std::collections::HashSet;
+use std::str::FromStr;
+use std::sync::Arc;
+use tauri::{Emitter, Manager};
+use tokio::sync::RwLock;
+
+/// 故障转移切换管理器
+///
+/// 负责处理故障转移成功后的供应商切换，确保 UI 能够直观反映当前使用的供应商。
+#[derive(Clone)]
+pub struct FailoverSwitchManager {
+    /// 正在处理中的切换（key = "app_type:provider_id"）
+    pending_switches: Arc<RwLock<HashSet<String>>>,
+    db: Arc<Database>,
+}
+
+impl FailoverSwitchManager {
+    pub fn new(db: Arc<Database>) -> Self {
+        Self {
+            pending_switches: Arc::new(RwLock::new(HashSet::new())),
+            db,
+        }
+    }
+
+    /// 尝试执行故障转移切换
+    ///
+    /// 如果相同的切换已在进行中，则跳过；否则执行切换逻辑。
+    ///
+    /// # Returns
+    /// - `Ok(true)` - 切换成功执行
+    /// - `Ok(false)` - 切换已在进行中，跳过
+    /// - `Err(e)` - 切换过程中发生错误
+    pub async fn try_switch(
+        &self,
+        app_handle: Option<&tauri::AppHandle>,
+        app_type: &str,
+        provider_id: &str,
+        provider_name: &str,
+    ) -> Result<bool, AppError> {
+        let switch_key = format!("{}:{}", app_type, provider_id);
+
+        // 去重检查：如果相同切换已在进行中，跳过
+        {
+            let mut pending = self.pending_switches.write().await;
+            if pending.contains(&switch_key) {
+                log::debug!(
+                    "[Failover] 切换已在进行中，跳过: {} -> {}",
+                    app_type,
+                    provider_id
+                );
+                return Ok(false);
+            }
+            pending.insert(switch_key.clone());
+        }
+
+        // 执行切换（确保最后清理 pending 标记）
+        let result = self
+            .do_switch(app_handle, app_type, provider_id, provider_name)
+            .await;
+
+        // 清理 pending 标记
+        {
+            let mut pending = self.pending_switches.write().await;
+            pending.remove(&switch_key);
+        }
+
+        result
+    }
+
+    async fn do_switch(
+        &self,
+        app_handle: Option<&tauri::AppHandle>,
+        app_type: &str,
+        provider_id: &str,
+        provider_name: &str,
+    ) -> Result<bool, AppError> {
+        log::info!(
+            "[Failover] 开始切换供应商: {} -> {} ({})",
+            app_type,
+            provider_name,
+            provider_id
+        );
+
+        // 1. 更新数据库 is_current
+        self.db.set_current_provider(app_type, provider_id)?;
+
+        // 2. 更新本地 settings（设备级）
+        let app_type_enum = crate::app_config::AppType::from_str(app_type)
+            .map_err(|_| AppError::Message(format!("无效的应用类型: {}", app_type)))?;
+        crate::settings::set_current_provider(&app_type_enum, Some(provider_id))?;
+
+        // 3. 更新托盘菜单和发射事件
+        if let Some(app) = app_handle {
+            // 更新托盘菜单
+            if let Some(app_state) = app.try_state::<crate::store::AppState>() {
+                // 更新 Live 备份（确保代理停止时恢复正确配置）
+                if let Ok(Some(provider)) = self.db.get_provider_by_id(provider_id, app_type) {
+                    if let Err(e) = app_state
+                        .proxy_service
+                        .update_live_backup_from_provider(app_type, &provider)
+                        .await
+                    {
+                        log::warn!("[Failover] 更新 Live 备份失败: {e}");
+                    }
+                }
+
+                // 重建托盘菜单
+                if let Ok(new_menu) = crate::tray::create_tray_menu(app, app_state.inner()) {
+                    if let Some(tray) = app.tray_by_id("main") {
+                        if let Err(e) = tray.set_menu(Some(new_menu)) {
+                            log::error!("[Failover] 更新托盘菜单失败: {e}");
+                        }
+                    }
+                }
+            }
+
+            // 发射事件到前端
+            let event_data = serde_json::json!({
+                "appType": app_type,
+                "providerId": provider_id,
+                "source": "failover"  // 标识来源是故障转移
+            });
+            if let Err(e) = app.emit("provider-switched", event_data) {
+                log::error!("[Failover] 发射供应商切换事件失败: {e}");
+            }
+        }
+
+        log::info!(
+            "[Failover] 供应商切换完成: {} -> {} ({})",
+            app_type,
+            provider_name,
+            provider_id
+        );
+
+        Ok(true)
+    }
+}