fix(dashboard): 修复仪表盘今日统计使用UTC时区的问题

将仪表盘统计中的"今日"时间范围从UTC时区改为服务器配置时区，
使其与使用记录页面保持一致。

修改内容：
- GetDashboardStats: 使用 timezone.Now() 和 timezone.Today()
- GetDashboardStatsWithRange: 同上

影响的统计项：
- 今日请求 (TodayRequests)
- 今日 Token (TodayTokens)
- 今日费用 (TodayCost/TodayActualCost)
- 今日新用户 (TodayNewUsers)
- 今日活跃用户 (ActiveUsers)

backend/internal/repository/usage_log_repo.go

@@ -270,13 +270,13 @@ type DashboardStats = usagestats.DashboardStats
 
 func (r *usageLogRepository) GetDashboardStats(ctx context.Context) (*DashboardStats, error) {
 	stats := &DashboardStats{}
-	now := time.Now().UTC()
+	now := timezone.Now()
-	todayUTC := truncateToDayUTC(now)
+	todayStart := timezone.Today()
 
-	if err := r.fillDashboardEntityStats(ctx, stats, todayUTC, now); err != nil {
+	if err := r.fillDashboardEntityStats(ctx, stats, todayStart, now); err != nil {
 		return nil, err
 	}
-	if err := r.fillDashboardUsageStatsAggregated(ctx, stats, todayUTC, now); err != nil {
+	if err := r.fillDashboardUsageStatsAggregated(ctx, stats, todayStart, now); err != nil {
 		return nil, err
 	}
 
@@ -298,13 +298,13 @@ func (r *usageLogRepository) GetDashboardStatsWithRange(ctx context.Context, sta
 	}
 
 	stats := &DashboardStats{}
-	now := time.Now().UTC()
+	now := timezone.Now()
-	todayUTC := truncateToDayUTC(now)
+	todayStart := timezone.Today()
 
-	if err := r.fillDashboardEntityStats(ctx, stats, todayUTC, now); err != nil {
+	if err := r.fillDashboardEntityStats(ctx, stats, todayStart, now); err != nil {
 		return nil, err
 	}
-	if err := r.fillDashboardUsageStatsFromUsageLogs(ctx, stats, startUTC, endUTC, todayUTC, now); err != nil {
+	if err := r.fillDashboardUsageStatsFromUsageLogs(ctx, stats, startUTC, endUTC, todayStart, now); err != nil {
 		return nil, err
 	}