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/pages/SettingsPage.tsx

@@ -11,6 +11,7 @@ import { useAdminAccess } from '../core/auth/useAdminAccess';
 import { useProfile, useUpdateProfile } from '../features/settings/hooks/useProfile';
 import { useExportUserData } from '../features/settings/hooks/useExportUserData';
 import { useVehicles } from '../features/vehicles/hooks/useVehicles';
+import { getVehicleSubtitle } from '@/core/utils/vehicleDisplay';
 import { useSubscription } from '../features/subscription/hooks/useSubscription';
 import { useTheme } from '../shared-minimal/theme/ThemeContext';
 import { DeleteAccountDialog } from '../features/settings/components/DeleteAccountDialog';
@@ -375,7 +376,7 @@ export const SettingsPage: React.FC = () => {
                   {index > 0 && <Divider />}
                   <ListItem sx={{ py: 1.5 }}>
                     <ListItemText
-                      primary={`${vehicle.year} ${vehicle.make} ${vehicle.model}`}
+                      primary={getVehicleSubtitle(vehicle) || 'Unknown Vehicle'}
                       secondary={vehicle.nickname || undefined}
                       primaryTypographyProps={{ fontWeight: 500 }}
                     />