feat(dashboard): add group usage distribution chart to usage page

Add a doughnut chart showing usage statistics broken down by group on
the admin usage records page. The chart appears alongside the existing
model distribution chart (2-column grid), with the token usage trend
chart moved to a separate full-width row below.

Changes:
- backend/pkg/usagestats: add GroupStat type
- backend/service: add GetGroupStatsWithFilters interface method and implementation
- backend/repository: implement GetGroupStatsWithFilters with LEFT JOIN groups
- backend/handler: add GetGroupStats handler with full filter support
- backend/routes: register GET /admin/dashboard/groups route
- backend/tests: add GetGroupStatsWithFilters stubs to contract/sora tests
- frontend/types: add GroupStat interface
- frontend/api: add getGroupStats API function and types
- frontend/components: add GroupDistributionChart.vue doughnut chart
- frontend/views: update UsageView layout and load group stats in parallel
- frontend/i18n: add groupDistribution, group, noGroup keys (zh + en)

backend/internal/repository/usage_log_repo.go

@@ -1784,6 +1784,77 @@ func (r *usageLogRepository) GetModelStatsWithFilters(ctx context.Context, start
 	return results, nil
 }
 
+// GetGroupStatsWithFilters returns group usage statistics with optional filters
+func (r *usageLogRepository) GetGroupStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, requestType *int16, stream *bool, billingType *int8) (results []usagestats.GroupStat, err error) {
+	query := `
+		SELECT
+			COALESCE(ul.group_id, 0) as group_id,
+			COALESCE(g.name, '') as group_name,
+			COUNT(*) as requests,
+			COALESCE(SUM(ul.input_tokens + ul.output_tokens + ul.cache_creation_tokens + ul.cache_read_tokens), 0) as total_tokens,
+			COALESCE(SUM(ul.total_cost), 0) as cost,
+			COALESCE(SUM(ul.actual_cost), 0) as actual_cost
+		FROM usage_logs ul
+		LEFT JOIN groups g ON g.id = ul.group_id
+		WHERE ul.created_at >= $1 AND ul.created_at < $2
+	`
+
+	args := []any{startTime, endTime}
+	if userID > 0 {
+		query += fmt.Sprintf(" AND ul.user_id = $%d", len(args)+1)
+		args = append(args, userID)
+	}
+	if apiKeyID > 0 {
+		query += fmt.Sprintf(" AND ul.api_key_id = $%d", len(args)+1)
+		args = append(args, apiKeyID)
+	}
+	if accountID > 0 {
+		query += fmt.Sprintf(" AND ul.account_id = $%d", len(args)+1)
+		args = append(args, accountID)
+	}
+	if groupID > 0 {
+		query += fmt.Sprintf(" AND ul.group_id = $%d", len(args)+1)
+		args = append(args, groupID)
+	}
+	query, args = appendRequestTypeOrStreamQueryFilter(query, args, requestType, stream)
+	if billingType != nil {
+		query += fmt.Sprintf(" AND ul.billing_type = $%d", len(args)+1)
+		args = append(args, int16(*billingType))
+	}
+	query += " GROUP BY ul.group_id, g.name ORDER BY total_tokens DESC"
+
+	rows, err := r.sql.QueryContext(ctx, query, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if closeErr := rows.Close(); closeErr != nil && err == nil {
+			err = closeErr
+			results = nil
+		}
+	}()
+
+	results = make([]usagestats.GroupStat, 0)
+	for rows.Next() {
+		var row usagestats.GroupStat
+		if err := rows.Scan(
+			&row.GroupID,
+			&row.GroupName,
+			&row.Requests,
+			&row.TotalTokens,
+			&row.Cost,
+			&row.ActualCost,
+		); err != nil {
+			return nil, err
+		}
+		results = append(results, row)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return results, nil
+}
+
 // GetGlobalStats gets usage statistics for all users within a time range
 func (r *usageLogRepository) GetGlobalStats(ctx context.Context, startTime, endTime time.Time) (*UsageStats, error) {
 	query := `