fix: billing mode display follows cost calculation result

Instead of hardcoding BillingMode="image" when ImageCount>0,
let cost.BillingMode (set by CalculateCostUnified/CalculateImageCost)
take priority. This ensures channel token pricing shows "token" mode.

backend/internal/service/billing_service.go

@@ -890,8 +890,9 @@ func (s *BillingService) CalculateImageCost(model string, imageSize string, imag
 	actualCost := totalCost * rateMultiplier
 
 	return &CostBreakdown{
-		TotalCost:  totalCost,
-		ActualCost: actualCost,
+		TotalCost:   totalCost,
+		ActualCost:  actualCost,
+		BillingMode: string(BillingModeImage),
 	}
 }