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/proxy/server.rs

@@ -190,6 +190,18 @@ impl ProxyServer {
         status
     }
 
+    /// 更新某个应用类型当前“目标供应商”（用于 UI 展示 active_targets）
+    ///
+    /// 注意：这不代表该供应商一定已经处理过请求，而是用于“热切换/启用故障转移立即切 P1”
+    /// 等场景下，让 UI 能立刻反映最新目标。
+    pub async fn set_active_target(&self, app_type: &str, provider_id: &str, provider_name: &str) {
+        let mut current_providers = self.state.current_providers.write().await;
+        current_providers.insert(
+            app_type.to_string(),
+            (provider_id.to_string(), provider_name.to_string()),
+        );
+    }
+
     fn build_router(&self) -> Router {
         let cors = CorsLayer::new()
             .allow_origin(Any)