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/types.ts

@@ -23,8 +23,6 @@ export interface Provider {
   // 图标配置
   icon?: string; // 图标名称（如 "openai", "anthropic"）
   iconColor?: string; // 图标颜色（Hex 格式，如 "#00A67E"）
-  // 新增：是否为代理目标
-  isProxyTarget?: boolean;
 }
 
 export interface AppConfig {