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/pages/VehicleDetailPage.tsx

@@ -12,6 +12,7 @@ import LocalGasStationIcon from '@mui/icons-material/LocalGasStation';
 import BuildIcon from '@mui/icons-material/Build';
 import DeleteIcon from '@mui/icons-material/Delete';
 import { Vehicle } from '../types/vehicles.types';
+import { getVehicleLabel, getVehicleSubtitle } from '@/core/utils/vehicleDisplay';
 import { vehiclesApi } from '../api/vehicles.api';
 import { Card } from '../../../shared-minimal/components/Card';
 import { VehicleForm } from '../components/VehicleForm';
@@ -224,8 +225,7 @@ export const VehicleDetailPage: React.FC = () => {
     );
   }
 
-  const displayName = vehicle.nickname || 
-    [vehicle.year, vehicle.make, vehicle.model, vehicle.trimLevel].filter(Boolean).join(' ') || 'Vehicle';
+  const displayName = getVehicleLabel(vehicle);
 
   const handleRowClick = (recId: string, type: VehicleRecord['type']) => {
     if (type === 'Fuel Logs') {
@@ -373,8 +373,7 @@ export const VehicleDetailPage: React.FC = () => {
               Vehicle Details
             </Typography>
             <Typography variant="body2" color="text.secondary">
-              {vehicle.year} {vehicle.make} {vehicle.model}
-              {vehicle.trimLevel && ` ${vehicle.trimLevel}`}
+              {getVehicleSubtitle(vehicle) || 'Unknown Vehicle'}
             </Typography>
             {vehicle.vin && (
               <Typography variant="body2" color="text.secondary" sx={{ mt: 0.5 }}>