fix: display purchase info and fix validation on vehicle detail (refs #41)

- Add purchase price and purchase date display to vehicle detail page
- Fix form validation to handle NaN from empty number inputs using z.preprocess

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/features/vehicles/components/VehicleForm.tsx

@@ -13,10 +13,13 @@ import { VehicleImageUpload } from './VehicleImageUpload';
 import { useTierAccess } from '../../../core/hooks/useTierAccess';
 import { UpgradeRequiredDialog } from '../../../shared-minimal/components/UpgradeRequiredDialog';
 
+// Helper to convert NaN (from empty number inputs) to null
+const nanToNull = (val: unknown) => (typeof val === 'number' && isNaN(val) ? null : val);
+
 const vehicleSchema = z
   .object({
     vin: z.string().max(17).nullable().optional().transform(val => val ?? undefined),
-    year: z.number().min(1950).max(new Date().getFullYear() + 1).nullable().optional(),
+    year: z.preprocess(nanToNull, z.number().min(1950).max(new Date().getFullYear() + 1).nullable().optional()),
     make: z.string().nullable().optional(),
     model: z.string().nullable().optional(),
     engine: z.string().nullable().optional(),
@@ -27,8 +30,8 @@ const vehicleSchema = z
     nickname: z.string().nullable().optional(),
     color: z.string().nullable().optional(),
     licensePlate: z.string().nullable().optional(),
-    odometerReading: z.number().min(0).nullable().optional(),
-    purchasePrice: z.number().min(0).nullable().optional(),
+    odometerReading: z.preprocess(nanToNull, z.number().min(0).nullable().optional()),
+    purchasePrice: z.preprocess(nanToNull, z.number().min(0).nullable().optional()),
     purchaseDate: z.string().nullable().optional(),
   })
   .refine(