feat(proxy): implement independent failover queue management

Add a new failover queue system that operates independently from provider
sortIndex, allowing users to configure failover order per app type.

Backend changes:
- Add failover_queue table to schema.rs for persistent storage
- Create dao/failover.rs with CRUD operations for queue management
- Add Tauri commands for queue operations (get, add, remove, reorder, toggle)
- Refactor provider_router.rs select_providers() to use failover queue:
  - Current provider always takes first priority
  - Queue providers ordered by queue_order as fallback
  - Only providers with open circuit breakers are included

Frontend changes:
- Add FailoverQueueItem type to proxy.ts
- Extend failover.ts API with queue management methods
- Add React Query hooks for queue data fetching and mutations
- Create FailoverQueueManager component with drag-and-drop reordering
- Integrate queue management into SettingsPage under "Auto Failover"
- Add i18n translations for zh and en locales

src-tauri/src/database/mod.rs

@@ -31,6 +31,9 @@ mod schema;
 #[cfg(test)]
 mod tests;
 
+// DAO 类型导出供外部使用
+pub use dao::FailoverQueueItem;
+
 use crate::config::get_app_config_dir;
 use crate::error::AppError;
 use rusqlite::Connection;