refactor(antigravity): 统一额度刷新机制与 Claude 一致

将 Antigravity 的额度刷新从后台定时刷新改为按需获取模式，与 Claude 统一：

- 删除 AntigravityQuotaRefresher 后台服务
- 新增 QuotaFetcher 接口和 AntigravityQuotaFetcher 实现
- 前端改为调用 usage API 获取额度，支持 loading/error 状态
- 统一使用内存缓存（10 分钟 TTL）

backend/internal/service/wire.go

@@ -54,18 +54,6 @@ func ProvideTimingWheelService() *TimingWheelService {
 	return svc
 }
 
-// ProvideAntigravityQuotaRefresher creates and starts AntigravityQuotaRefresher
-func ProvideAntigravityQuotaRefresher(
-	accountRepo AccountRepository,
-	proxyRepo ProxyRepository,
-	oauthSvc *AntigravityOAuthService,
-	cfg *config.Config,
-) *AntigravityQuotaRefresher {
-	svc := NewAntigravityQuotaRefresher(accountRepo, proxyRepo, oauthSvc, cfg)
-	svc.Start()
-	return svc
-}
-
 // ProvideDeferredService creates and starts DeferredService
 func ProvideDeferredService(accountRepo AccountRepository, timingWheel *TimingWheelService) *DeferredService {
 	svc := NewDeferredService(accountRepo, timingWheel, 10*time.Second)
@@ -124,6 +112,6 @@ var ProviderSet = wire.NewSet(
 	ProvideTokenRefreshService,
 	ProvideTimingWheelService,
 	ProvideDeferredService,
-	ProvideAntigravityQuotaRefresher,
+	NewAntigravityQuotaFetcher,
 	NewUserAttributeService,
 )