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/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 });
   },