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/schema.rs

@@ -319,6 +319,30 @@ impl Database {
             [],
         );
 
+        // 14. Failover Queue 表 (故障转移队列)
+        conn.execute(
+            "CREATE TABLE IF NOT EXISTS failover_queue (
+                id INTEGER PRIMARY KEY AUTOINCREMENT,
+                app_type TEXT NOT NULL,
+                provider_id TEXT NOT NULL,
+                queue_order INTEGER NOT NULL,
+                enabled INTEGER NOT NULL DEFAULT 1,
+                created_at INTEGER NOT NULL,
+                UNIQUE (app_type, provider_id),
+                FOREIGN KEY (provider_id, app_type) REFERENCES providers(id, app_type) ON DELETE CASCADE
+            )",
+            [],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+
+        // 为故障转移队列创建索引
+        conn.execute(
+            "CREATE INDEX IF NOT EXISTS idx_failover_queue_order
+             ON failover_queue(app_type, queue_order)",
+            [],
+        )
+        .map_err(|e| AppError::Database(e.to_string()))?;
+
         Ok(())
     }