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/email-ingestion/components/ResolveAssociationDialog.tsx

@@ -23,6 +23,7 @@ import CheckCircleRoundedIcon from '@mui/icons-material/CheckCircleRounded';
 import { useVehicles } from '../../vehicles/hooks/useVehicles';
 import { useResolveAssociation } from '../hooks/usePendingAssociations';
 import type { PendingVehicleAssociation } from '../types/email-ingestion.types';
+import { getVehicleLabel } from '@/core/utils/vehicleDisplay';
 
 interface ResolveAssociationDialogProps {
   open: boolean;
@@ -166,9 +167,7 @@ export const ResolveAssociationDialog: React.FC<ResolveAssociationDialogProps> =
               <Box sx={{ display: 'flex', flexDirection: 'column', gap: 1 }}>
                 {vehicles.map((vehicle) => {
                   const isSelected = selectedVehicleId === vehicle.id;
-                  const vehicleName = vehicle.nickname
-                    || [vehicle.year, vehicle.make, vehicle.model].filter(Boolean).join(' ')
-                    || 'Unnamed Vehicle';
+                  const vehicleName = getVehicleLabel(vehicle);
 
                   return (
                     <Box