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/dashboard/components/VehicleAttention.tsx

@@ -9,6 +9,7 @@ import ErrorRoundedIcon from '@mui/icons-material/ErrorRounded';
 import WarningAmberRoundedIcon from '@mui/icons-material/WarningAmberRounded';
 import ScheduleRoundedIcon from '@mui/icons-material/ScheduleRounded';
 import { GlassCard } from '../../../shared-minimal/components/mobile/GlassCard';
+import { getVehicleLabel } from '@/core/utils/vehicleDisplay';
 import { VehicleNeedingAttention } from '../types';
 
 interface VehicleAttentionProps {
@@ -104,7 +105,7 @@ export const VehicleAttention: React.FC<VehicleAttentionProps> = ({ vehicles, on
                       mb: 0.5,
                     }}
                   >
-                    {vehicle.nickname || `${vehicle.year} ${vehicle.make} ${vehicle.model}`}
+                    {getVehicleLabel(vehicle)}
                   </Box>
                   <p className="text-sm text-slate-600 dark:text-titanio">
                     {vehicle.reason}