refactor(proxy): remove is_proxy_target in favor of failover_queue

- Remove `is_proxy_target` field from Provider struct (Rust & TypeScript)
- Remove related DAO methods: get_proxy_target_provider, set_proxy_target
- Remove deprecated Tauri commands: get_proxy_targets, set_proxy_target
- Add `is_available()` method to CircuitBreaker for availability checks
  without consuming HalfOpen probe permits (used in select_providers)
- Keep `allow_request()` for actual request gating with permit tracking
- Update stream_check to use failover_queue instead of is_proxy_target
- Clean up commented-out reset circuit breaker button in ProviderActions
- Remove unused useProxyTargets and useSetProxyTarget hooks

src/lib/api/failover.ts

@@ -18,26 +18,9 @@ export interface Provider {
   meta?: unknown;
   icon?: string;
   iconColor?: string;
-  isProxyTarget?: boolean;
 }
 
 export const failoverApi = {
-  // ========== 旧版代理目标 API（保留向后兼容）==========
-
-  // 获取代理目标列表
-  async getProxyTargets(appType: string): Promise<Provider[]> {
-    return invoke("get_proxy_targets", { appType });
-  },
-
-  // 设置代理目标
-  async setProxyTarget(
-    providerId: string,
-    appType: string,
-    enabled: boolean,
-  ): Promise<void> {
-    return invoke("set_proxy_target", { providerId, appType, enabled });
-  },
-
   // ========== 熔断器 API ==========
 
   // 获取供应商健康状态

src/lib/api/providers.ts

@@ -38,10 +38,6 @@ export const providersApi = {
     return await invoke("switch_provider", { id, app: appId });
   },
 
-  async setProxyTarget(id: string, appId: AppId): Promise<boolean> {
-    return await invoke("set_proxy_target_provider", { id, app: appId });
-  },
-
   async importDefault(appId: AppId): Promise<boolean> {
     return await invoke("import_default_config", { app: appId });
   },