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/antigravity_quota_fetcher.go

@@ -0,0 +1,111 @@
+package service
+
+import (
+	"context"
+	"time"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/antigravity"
+)
+
+// AntigravityQuotaFetcher 从 Antigravity API 获取额度
+type AntigravityQuotaFetcher struct {
+	proxyRepo ProxyRepository
+}
+
+// NewAntigravityQuotaFetcher 创建 AntigravityQuotaFetcher
+func NewAntigravityQuotaFetcher(proxyRepo ProxyRepository) *AntigravityQuotaFetcher {
+	return &AntigravityQuotaFetcher{proxyRepo: proxyRepo}
+}
+
+// CanFetch 检查是否可以获取此账户的额度
+func (f *AntigravityQuotaFetcher) CanFetch(account *Account) bool {
+	if account.Platform != PlatformAntigravity {
+		return false
+	}
+	accessToken := account.GetCredential("access_token")
+	return accessToken != ""
+}
+
+// FetchQuota 获取 Antigravity 账户额度信息
+func (f *AntigravityQuotaFetcher) FetchQuota(ctx context.Context, account *Account, proxyURL string) (*QuotaResult, error) {
+	accessToken := account.GetCredential("access_token")
+	projectID := account.GetCredential("project_id")
+
+	// 如果没有 project_id，生成一个随机的
+	if projectID == "" {
+		projectID = antigravity.GenerateMockProjectID()
+	}
+
+	client := antigravity.NewClient(proxyURL)
+
+	// 调用 API 获取配额
+	modelsResp, modelsRaw, err := client.FetchAvailableModels(ctx, accessToken, projectID)
+	if err != nil {
+		return nil, err
+	}
+
+	// 转换为 UsageInfo
+	usageInfo := f.buildUsageInfo(modelsResp)
+
+	return &QuotaResult{
+		UsageInfo: usageInfo,
+		Raw:       modelsRaw,
+	}, nil
+}
+
+// buildUsageInfo 将 API 响应转换为 UsageInfo
+func (f *AntigravityQuotaFetcher) buildUsageInfo(modelsResp *antigravity.FetchAvailableModelsResponse) *UsageInfo {
+	now := time.Now()
+	info := &UsageInfo{
+		UpdatedAt:        &now,
+		AntigravityQuota: make(map[string]*AntigravityModelQuota),
+	}
+
+	// 遍历所有模型，填充 AntigravityQuota
+	for modelName, modelInfo := range modelsResp.Models {
+		if modelInfo.QuotaInfo == nil {
+			continue
+		}
+
+		// remainingFraction 是剩余比例 (0.0-1.0)，转换为使用率百分比
+		utilization := int((1.0 - modelInfo.QuotaInfo.RemainingFraction) * 100)
+
+		info.AntigravityQuota[modelName] = &AntigravityModelQuota{
+			Utilization: utilization,
+			ResetTime:   modelInfo.QuotaInfo.ResetTime,
+		}
+	}
+
+	// 同时设置 FiveHour 用于兼容展示（取主要模型）
+	priorityModels := []string{"claude-sonnet-4-20250514", "claude-sonnet-4", "gemini-2.5-pro"}
+	for _, modelName := range priorityModels {
+		if modelInfo, ok := modelsResp.Models[modelName]; ok && modelInfo.QuotaInfo != nil {
+			utilization := (1.0 - modelInfo.QuotaInfo.RemainingFraction) * 100
+			progress := &UsageProgress{
+				Utilization: utilization,
+			}
+			if modelInfo.QuotaInfo.ResetTime != "" {
+				if resetTime, err := time.Parse(time.RFC3339, modelInfo.QuotaInfo.ResetTime); err == nil {
+					progress.ResetsAt = &resetTime
+					progress.RemainingSeconds = int(time.Until(resetTime).Seconds())
+				}
+			}
+			info.FiveHour = progress
+			break
+		}
+	}
+
+	return info
+}
+
+// GetProxyURL 获取账户的代理 URL
+func (f *AntigravityQuotaFetcher) GetProxyURL(ctx context.Context, account *Account) string {
+	if account.ProxyID == nil || f.proxyRepo == nil {
+		return ""
+	}
+	proxy, err := f.proxyRepo.GetByID(ctx, *account.ProxyID)
+	if err != nil || proxy == nil {
+		return ""
+	}
+	return proxy.URL()
+}