fix: promote vehicle display utils to core with null safety (refs #165)

Create shared getVehicleLabel/getVehicleSubtitle in core/utils with
VehicleLike interface. Replace all direct year/make/model concatenation
across 17 consumer files to prevent null values in vehicle names.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/features/vehicles/components/VehicleCard.tsx

@@ -9,6 +9,7 @@ import DeleteIcon from '@mui/icons-material/Delete';
 import { Vehicle } from '../types/vehicles.types';
 import { useUnits } from '../../../core/units/UnitsContext';
 import { VehicleImage } from './VehicleImage';
+import { getVehicleLabel } from '@/core/utils/vehicleDisplay';
 
 interface VehicleCardProps {
   vehicle: Vehicle;
@@ -24,8 +25,7 @@ export const VehicleCard: React.FC<VehicleCardProps> = ({
   onSelect,
 }) => {
   const { formatDistance } = useUnits();
-  const displayName = vehicle.nickname || 
-    [vehicle.year, vehicle.make, vehicle.model, vehicle.trimLevel].filter(Boolean).join(' ');
+  const displayName = getVehicleLabel(vehicle);
 
   return (
     <Card