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/subscription/components/VehicleSelectionDialog.tsx

@@ -13,6 +13,7 @@ import {
   Box,
 } from '@mui/material';
 import type { SubscriptionTier } from '../types/subscription.types';
+import { getVehicleLabel } from '@/core/utils/vehicleDisplay';
 
 interface Vehicle {
   id: string;
@@ -70,13 +71,6 @@ export const VehicleSelectionDialog = ({
     onConfirm(selectedVehicleIds);
   };
 
-  const getVehicleLabel = (vehicle: Vehicle): string => {
-    if (vehicle.nickname) {
-      return vehicle.nickname;
-    }
-    const parts = [vehicle.year, vehicle.make, vehicle.model].filter(Boolean);
-    return parts.join(' ') || 'Unknown Vehicle';
-  };
 
   const canConfirm = selectedVehicleIds.length > 0 && selectedVehicleIds.length <= maxSelections;