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/pages/VehicleDetailPage.tsx

@@ -414,6 +414,23 @@ export const VehicleDetailPage: React.FC = () => {
             label="Current Odometer Reading"
             value={vehicle.odometerReading ? `${vehicle.odometerReading.toLocaleString()} mi` : undefined}
           />
+
+          {/* Purchase Information Section */}
+          <div className="border-t border-gray-200 dark:border-silverstone pt-4 mt-4">
+            <h3 className="text-lg font-medium text-gray-900 dark:text-avus mb-4">
+              Purchase Information
+            </h3>
+            <div className="grid grid-cols-2 gap-4">
+              <DetailField
+                label="Purchase Price"
+                value={vehicle.purchasePrice ? `$${vehicle.purchasePrice.toLocaleString('en-US', { minimumFractionDigits: 2, maximumFractionDigits: 2 })}` : undefined}
+              />
+              <DetailField
+                label="Purchase Date"
+                value={vehicle.purchaseDate ? new Date(vehicle.purchaseDate).toLocaleDateString() : undefined}
+              />
+            </div>
+          </div>
         </form>
 
         <Divider sx={{ my: 4 }} />