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/core/utils/vehicleDisplay.ts

@@ -0,0 +1,27 @@
+/** Vehicle-like object with minimal fields for display purposes */
+export interface VehicleLike {
+  year?: number | null;
+  make?: string | null;
+  model?: string | null;
+  trimLevel?: string | null;
+  nickname?: string | null;
+  vin?: string | null;
+  id?: string | null;
+}
+
+/** Primary display name with fallback chain: nickname -> year/make/model -> VIN -> ID */
+export const getVehicleLabel = (vehicle: VehicleLike | undefined): string => {
+  if (!vehicle) return 'Unknown Vehicle';
+  if (vehicle.nickname?.trim()) return vehicle.nickname.trim();
+  const parts = [vehicle.year, vehicle.make, vehicle.model, vehicle.trimLevel].filter(Boolean);
+  if (parts.length > 0) return parts.join(' ');
+  if (vehicle.vin) return vehicle.vin;
+  return vehicle.id ? `${vehicle.id.substring(0, 8)}...` : 'Unknown Vehicle';
+};
+
+/** Subtitle line: "Year Make Model" with null safety. Returns empty string if insufficient data. */
+export const getVehicleSubtitle = (vehicle: VehicleLike | undefined): string => {
+  if (!vehicle) return '';
+  const parts = [vehicle.year?.toString(), vehicle.make, vehicle.model].filter(Boolean);
+  return parts.length >= 2 ? parts.join(' ') : '';
+};