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/settings/mobile/MobileSettingsScreen.tsx

@@ -7,6 +7,7 @@ import { useSettings } from '../hooks/useSettings';
 import { useProfile, useUpdateProfile } from '../hooks/useProfile';
 import { useExportUserData } from '../hooks/useExportUserData';
 import { useVehicles } from '../../vehicles/hooks/useVehicles';
+import { getVehicleSubtitle } from '@/core/utils/vehicleDisplay';
 import { useSubscription } from '../../subscription/hooks/useSubscription';
 import { useAdminAccess } from '../../../core/auth/useAdminAccess';
 import { useNavigationStore } from '../../../core/store';
@@ -373,7 +374,7 @@ export const MobileSettingsScreen: React.FC = () => {
                     className="p-3 bg-slate-50 dark:bg-nero rounded-lg"
                   >
                     <p className="font-medium text-slate-800 dark:text-avus">
-                      {vehicle.year} {vehicle.make} {vehicle.model}
+                      {getVehicleSubtitle(vehicle) || 'Unknown Vehicle'}
                     </p>
                     {vehicle.nickname && (
                       <p className="text-sm text-slate-500 dark:text-titanio">{vehicle.nickname}</p>