refactor(frontend): 优化 Gemini 配额显示，参考 Antigravity 样式

- 简化标签：将 "RPD Pro/Flash" 改为 "Pro/Flash"，避免文字截断
- 添加账号类型徽章（Free/Pro/Ultra），带颜色区分
- 添加帮助图标（?），悬停显示限流政策和官方文档链接
- 重构显示布局：账号类型 + 两行配额（Pro/Flash）
- 移除冗余的 AccountQuotaInfo 组件调用

backend/internal/service/account_usage_service.go

@@ -202,22 +202,25 @@ func (s *AccountUsageService) GetUsage(ctx context.Context, accountID int64) (*U
 
 func (s *AccountUsageService) getGeminiUsage(ctx context.Context, account *Account) (*UsageInfo, error) {
 	now := time.Now()
-	start := geminiDailyWindowStart(now)
-
-	stats, err := s.usageLogRepo.GetModelStatsWithFilters(ctx, start, now, 0, 0, account.ID)
-	if err != nil {
-		return nil, fmt.Errorf("get gemini usage stats failed: %w", err)
-	}
-
 	usage := &UsageInfo{
 		UpdatedAt: &now,
 	}
 
+	if s.geminiQuotaService == nil || s.usageLogRepo == nil {
+		return usage, nil
+	}
+
 	quota, ok := s.geminiQuotaService.QuotaForAccount(ctx, account)
 	if !ok {
 		return usage, nil
 	}
 
+	start := geminiDailyWindowStart(now)
+	stats, err := s.usageLogRepo.GetModelStatsWithFilters(ctx, start, now, 0, 0, account.ID)
+	if err != nil {
+		return nil, fmt.Errorf("get gemini usage stats failed: %w", err)
+	}
+
 	totals := geminiAggregateUsage(stats)
 	resetAt := geminiDailyResetTime(now)