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/query/failover.ts

@@ -1,50 +1,6 @@
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { failoverApi } from "@/lib/api/failover";
 
-// ========== 旧版代理目标 Hooks（保留向后兼容）==========
-
-/**
- * 获取代理目标列表
- */
-export function useProxyTargets(appType: string) {
-  return useQuery({
-    queryKey: ["proxyTargets", appType],
-    queryFn: () => failoverApi.getProxyTargets(appType),
-    enabled: !!appType,
-  });
-}
-
-/**
- * 设置代理目标
- */
-export function useSetProxyTarget() {
-  const queryClient = useQueryClient();
-
-  return useMutation({
-    mutationFn: ({
-      providerId,
-      appType,
-      enabled,
-    }: {
-      providerId: string;
-      appType: string;
-      enabled: boolean;
-    }) => failoverApi.setProxyTarget(providerId, appType, enabled),
-    onSuccess: (_, variables) => {
-      // 刷新代理目标列表
-      queryClient.invalidateQueries({
-        queryKey: ["proxyTargets", variables.appType],
-      });
-      // 刷新供应商列表
-      queryClient.invalidateQueries({ queryKey: ["providers"] });
-      // 刷新健康状态
-      queryClient.invalidateQueries({
-        queryKey: ["providerHealth", variables.providerId, variables.appType],
-      });
-    },
-  });
-}
-
 // ========== 熔断器 Hooks ==========
 
 /**

src/lib/query/mutations.ts

@@ -180,35 +180,6 @@ export const useSwitchProviderMutation = (appId: AppId) => {
   });
 };
 
-export const useSetProxyTargetMutation = (appId: AppId) => {
-  const queryClient = useQueryClient();
-  const { t } = useTranslation();
-
-  return useMutation({
-    mutationFn: async (providerId: string) => {
-      return await providersApi.setProxyTarget(providerId, appId);
-    },
-    onSuccess: async () => {
-      await queryClient.invalidateQueries({ queryKey: ["providers", appId] });
-      toast.success(
-        t("notifications.proxyTargetSet", {
-          defaultValue: "已设置代理目标",
-        }),
-        { closeButton: true },
-      );
-    },
-    onError: (error: Error) => {
-      const detail = extractErrorMessage(error) || t("common.unknown");
-      toast.error(
-        t("notifications.setProxyTargetFailed", {
-          defaultValue: "设置代理目标失败: {{error}}",
-          error: detail,
-        }),
-      );
-    },
-  });
-};
-
 export const useSaveSettingsMutation = () => {
   const queryClient = useQueryClient();