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/account_usage_service.go

@@ -91,6 +91,12 @@ type UsageProgress struct {
 	WindowStats      *WindowStats `json:"window_stats,omitempty"` // 窗口期统计（从窗口开始到当前的使用量）
 }
 
+// AntigravityModelQuota Antigravity 单个模型的配额信息
+type AntigravityModelQuota struct {
+	Utilization int    `json:"utilization"` // 使用率 0-100
+	ResetTime   string `json:"reset_time"`  // 重置时间 ISO8601
+}
+
 // UsageInfo 账号使用量信息
 type UsageInfo struct {
 	UpdatedAt        *time.Time     `json:"updated_at,omitempty"`         // 更新时间
@@ -99,6 +105,9 @@ type UsageInfo struct {
 	SevenDaySonnet   *UsageProgress `json:"seven_day_sonnet,omitempty"`   // 7天Sonnet窗口
 	GeminiProDaily   *UsageProgress `json:"gemini_pro_daily,omitempty"`   // Gemini Pro 日配额
 	GeminiFlashDaily *UsageProgress `json:"gemini_flash_daily,omitempty"` // Gemini Flash 日配额
+
+	// Antigravity 多模型配额
+	AntigravityQuota map[string]*AntigravityModelQuota `json:"antigravity_quota,omitempty"`
 }
 
 // ClaudeUsageResponse Anthropic API返回的usage结构
@@ -124,19 +133,27 @@ type ClaudeUsageFetcher interface {
 
 // AccountUsageService 账号使用量查询服务
 type AccountUsageService struct {
-	accountRepo        AccountRepository
-	usageLogRepo       UsageLogRepository
-	usageFetcher       ClaudeUsageFetcher
-	geminiQuotaService *GeminiQuotaService
+	accountRepo              AccountRepository
+	usageLogRepo             UsageLogRepository
+	usageFetcher             ClaudeUsageFetcher
+	geminiQuotaService       *GeminiQuotaService
+	antigravityQuotaFetcher  *AntigravityQuotaFetcher
 }
 
 // NewAccountUsageService 创建AccountUsageService实例
-func NewAccountUsageService(accountRepo AccountRepository, usageLogRepo UsageLogRepository, usageFetcher ClaudeUsageFetcher, geminiQuotaService *GeminiQuotaService) *AccountUsageService {
+func NewAccountUsageService(
+	accountRepo AccountRepository,
+	usageLogRepo UsageLogRepository,
+	usageFetcher ClaudeUsageFetcher,
+	geminiQuotaService *GeminiQuotaService,
+	antigravityQuotaFetcher *AntigravityQuotaFetcher,
+) *AccountUsageService {
 	return &AccountUsageService{
-		accountRepo:        accountRepo,
-		usageLogRepo:       usageLogRepo,
-		usageFetcher:       usageFetcher,
-		geminiQuotaService: geminiQuotaService,
+		accountRepo:              accountRepo,
+		usageLogRepo:             usageLogRepo,
+		usageFetcher:             usageFetcher,
+		geminiQuotaService:       geminiQuotaService,
+		antigravityQuotaFetcher:  antigravityQuotaFetcher,
 	}
 }
 
@@ -154,6 +171,11 @@ func (s *AccountUsageService) GetUsage(ctx context.Context, accountID int64) (*U
 		return s.getGeminiUsage(ctx, account)
 	}
 
+	// Antigravity 平台：使用 AntigravityQuotaFetcher 获取额度
+	if account.Platform == PlatformAntigravity {
+		return s.getAntigravityUsage(ctx, account)
+	}
+
 	// 只有oauth类型账号可以通过API获取usage（有profile scope）
 	if account.CanGetUsage() {
 		var apiResp *ClaudeUsageResponse
@@ -230,6 +252,51 @@ func (s *AccountUsageService) getGeminiUsage(ctx context.Context, account *Accou
 	return usage, nil
 }
 
+// antigravityUsageCache 缓存 Antigravity 额度数据
+type antigravityUsageCache struct {
+	usageInfo *UsageInfo
+	timestamp time.Time
+}
+
+var antigravityCacheMap = sync.Map{}
+
+// getAntigravityUsage 获取 Antigravity 账户额度
+func (s *AccountUsageService) getAntigravityUsage(ctx context.Context, account *Account) (*UsageInfo, error) {
+	if s.antigravityQuotaFetcher == nil || !s.antigravityQuotaFetcher.CanFetch(account) {
+		now := time.Now()
+		return &UsageInfo{UpdatedAt: &now}, nil
+	}
+
+	// 1. 检查缓存（10 分钟）
+	if cached, ok := antigravityCacheMap.Load(account.ID); ok {
+		if cache, ok := cached.(*antigravityUsageCache); ok && time.Since(cache.timestamp) < apiCacheTTL {
+			// 重新计算 RemainingSeconds
+			usage := cache.usageInfo
+			if usage.FiveHour != nil && usage.FiveHour.ResetsAt != nil {
+				usage.FiveHour.RemainingSeconds = int(time.Until(*usage.FiveHour.ResetsAt).Seconds())
+			}
+			return usage, nil
+		}
+	}
+
+	// 2. 获取代理 URL
+	proxyURL := s.antigravityQuotaFetcher.GetProxyURL(ctx, account)
+
+	// 3. 调用 API 获取额度
+	result, err := s.antigravityQuotaFetcher.FetchQuota(ctx, account, proxyURL)
+	if err != nil {
+		return nil, fmt.Errorf("fetch antigravity quota failed: %w", err)
+	}
+
+	// 4. 缓存结果
+	antigravityCacheMap.Store(account.ID, &antigravityUsageCache{
+		usageInfo: result.UsageInfo,
+		timestamp: time.Now(),
+	})
+
+	return result.UsageInfo, nil
+}
+
 // addWindowStats 为 usage 数据添加窗口期统计
 // 使用独立缓存（1 分钟），与 API 缓存分离
 func (s *AccountUsageService) addWindowStats(ctx context.Context, account *Account, usage *UsageInfo) {