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/dao/mod.rs

@@ -2,6 +2,7 @@
 //!
 //! Database access operations for each domain
 
+pub mod failover;
 pub mod mcp;
 pub mod prompts;
 pub mod providers;
@@ -11,3 +12,5 @@ pub mod skills;
 pub mod stream_check;
 
 // 所有 DAO 方法都通过 Database impl 提供，无需单独导出
+// 导出 FailoverQueueItem 供外部使用
+pub use failover::FailoverQueueItem;