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/admin/mobile/AdminUsersMobileScreen.tsx

@@ -24,6 +24,7 @@ import {
   SubscriptionTier,
   ListUsersParams,
 } from '../types/admin.types';
+import { getVehicleSubtitle } from '@/core/utils/vehicleDisplay';
 
 // Modal component for dialogs
 interface ModalProps {
@@ -128,7 +129,7 @@ const UserVehiclesList: React.FC<{ auth0Sub: string; isOpen: boolean }> = ({ aut
         <div className="space-y-1">
           {data.vehicles.map((vehicle, idx) => (
             <div key={idx} className="text-sm text-slate-600 dark:text-silverstone bg-slate-50 dark:bg-carbon px-2 py-1 rounded">
-              {vehicle.year} {vehicle.make} {vehicle.model}
+              {getVehicleSubtitle(vehicle) || 'Unknown Vehicle'}
             </div>
           ))}
         </div>