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/i18n/locales/en.json

@@ -840,5 +840,50 @@
   },
   "agents": {
     "title": "Agents"
+  },
+  "proxy": {
+    "failoverQueue": {
+      "title": "Failover Queue",
+      "description": "Manage failover order for each app's providers",
+      "info": "The current active provider always takes priority. When requests fail, the system will try other providers in queue order.",
+      "selectProvider": "Select a provider to add to queue",
+      "noAvailableProviders": "No providers available to add",
+      "empty": "Failover queue is empty. Add providers to enable automatic failover.",
+      "dragHint": "Drag providers to adjust failover order. Lower numbers have higher priority.",
+      "toggleEnabled": "Enable/Disable",
+      "addSuccess": "Added to failover queue",
+      "addFailed": "Failed to add",
+      "removeSuccess": "Removed from failover queue",
+      "removeFailed": "Failed to remove",
+      "reorderSuccess": "Queue order updated",
+      "reorderFailed": "Failed to update order",
+      "toggleFailed": "Failed to update status"
+    },
+    "autoFailover": {
+      "info": "When multiple proxy targets are enabled, the system will try them in priority order. When a provider fails consecutively reaching the threshold, the circuit breaker will open and skip that provider.",
+      "configSaved": "Auto failover config saved",
+      "configSaveFailed": "Failed to save",
+      "retrySettings": "Retry & Timeout Settings",
+      "failureThreshold": "Failure Threshold",
+      "failureThresholdHint": "Open circuit breaker after this many consecutive failures (recommended: 3-10)",
+      "timeout": "Recovery Wait Time (seconds)",
+      "timeoutHint": "Wait this long before trying to recover after circuit opens (recommended: 30-120)",
+      "circuitBreakerSettings": "Circuit Breaker Advanced Settings",
+      "successThreshold": "Recovery Success Threshold",
+      "successThresholdHint": "Close circuit breaker after this many successes in half-open state",
+      "errorRate": "Error Rate Threshold (%)",
+      "errorRateHint": "Open circuit breaker when error rate exceeds this value",
+      "minRequests": "Minimum Requests",
+      "minRequestsHint": "Minimum requests before calculating error rate",
+      "explanationTitle": "How It Works",
+      "failureThresholdLabel": "Failure Threshold",
+      "failureThresholdExplain": "Circuit breaker opens after this many consecutive failures, making the provider temporarily unavailable",
+      "timeoutLabel": "Recovery Wait Time",
+      "timeoutExplain": "After circuit opens, wait this long before trying half-open state",
+      "successThresholdLabel": "Recovery Success Threshold",
+      "successThresholdExplain": "In half-open state, close circuit breaker after this many successes, making provider available again",
+      "errorRateLabel": "Error Rate Threshold",
+      "errorRateExplain": "Open circuit breaker when error rate exceeds this value, even if failure threshold not reached"
+    }
   }
 }

src/i18n/locales/zh.json

@@ -840,5 +840,50 @@
   },
   "agents": {
     "title": "智能体"
+  },
+  "proxy": {
+    "failoverQueue": {
+      "title": "故障转移队列",
+      "description": "管理各应用的供应商故障转移顺序",
+      "info": "当前激活的供应商始终优先。当请求失败时，系统会按队列顺序依次尝试其他供应商。",
+      "selectProvider": "选择供应商添加到队列",
+      "noAvailableProviders": "没有可添加的供应商",
+      "empty": "故障转移队列为空。添加供应商以启用自动故障转移。",
+      "dragHint": "拖拽供应商可调整故障转移顺序，序号越小优先级越高。",
+      "toggleEnabled": "启用/禁用",
+      "addSuccess": "已添加到故障转移队列",
+      "addFailed": "添加失败",
+      "removeSuccess": "已从故障转移队列移除",
+      "removeFailed": "移除失败",
+      "reorderSuccess": "队列顺序已更新",
+      "reorderFailed": "更新顺序失败",
+      "toggleFailed": "状态更新失败"
+    },
+    "autoFailover": {
+      "info": "当启用多个代理目标时，系统会按优先级顺序依次尝试。当某个供应商连续失败达到阈值时，熔断器会自动打开，跳过该供应商。",
+      "configSaved": "自动故障转移配置已保存",
+      "configSaveFailed": "保存失败",
+      "retrySettings": "重试与超时设置",
+      "failureThreshold": "失败阈值",
+      "failureThresholdHint": "连续失败多少次后打开熔断器（建议: 3-10）",
+      "timeout": "恢复等待时间（秒）",
+      "timeoutHint": "熔断器打开后，等待多久后尝试恢复（建议: 30-120）",
+      "circuitBreakerSettings": "熔断器高级设置",
+      "successThreshold": "恢复成功阈值",
+      "successThresholdHint": "半开状态下成功多少次后关闭熔断器",
+      "errorRate": "错误率阈值 (%)",
+      "errorRateHint": "错误率超过此值时打开熔断器",
+      "minRequests": "最小请求数",
+      "minRequestsHint": "计算错误率前的最小请求数",
+      "explanationTitle": "工作原理",
+      "failureThresholdLabel": "失败阈值",
+      "failureThresholdExplain": "连续失败达到此次数时，熔断器打开，该供应商暂时不可用",
+      "timeoutLabel": "恢复等待时间",
+      "timeoutExplain": "熔断器打开后，等待此时间后尝试半开状态",
+      "successThresholdLabel": "恢复成功阈值",
+      "successThresholdExplain": "半开状态下，成功达到此次数时关闭熔断器，供应商恢复可用",
+      "errorRateLabel": "错误率阈值",
+      "errorRateExplain": "错误率超过此值时，即使未达到失败阈值也会打开熔断器"
+    }
   }
 }