fix: remove license plate fallback from VIN field (refs #39)

The VIN Number field incorrectly showed license plate when VIN was empty.
Now displays "Not provided" for missing VIN values, matching mobile behavior.

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

frontend/src/features/vehicles/pages/VehicleDetailPage.tsx

@@ -387,8 +387,7 @@ export const VehicleDetailPage: React.FC = () => {
         <form className="space-y-4">
           <DetailField
             label="VIN Number"
-            value={vehicle.vin || vehicle.licensePlate}
-            isRequired
+            value={vehicle.vin}
           />
 
           {/* Vehicle Specification Section */}