feat(antigravity): 添加配额窗口显示功能

后端:
- 新增 AntigravityQuotaRefresher 定时刷新配额
- Client 添加 FetchAvailableModels 方法获取模型配额
- 配额数据存入 account.extra.quota 字段

前端:
- AccountUsageCell 支持显示 Antigravity 账户配额
- UsageProgressBar 新增 amber 颜色
- 显示 G3P/G3F/G3I/C4.5 四个配额进度条

backend/cmd/server/wire.go

@@ -71,6 +71,7 @@ func provideCleanup(
 	openaiOAuth *service.OpenAIOAuthService,
 	geminiOAuth *service.GeminiOAuthService,
 	antigravityOAuth *service.AntigravityOAuthService,
+	antigravityQuota *service.AntigravityQuotaRefresher,
 ) func() {
 	return func() {
 		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
@@ -109,6 +110,10 @@ func provideCleanup(
 				antigravityOAuth.Stop()
 				return nil
 			}},
+			{"AntigravityQuotaRefresher", func() error {
+				antigravityQuota.Stop()
+				return nil
+			}},
 			{"Redis", func() error {
 				return rdb.Close()
 			}},