Fix: Show Remove and Modify print area buttons for existing images

- Update PhotoPrintAreaField to accept existingImageUrl prop
- Display Remove and Modify print area buttons on edit page for existing images
- Users can now easily modify print area or remove existing images without
  uploading a new file

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

src/app/admin/products/[id]/edit/page.tsx

@@ -266,7 +266,7 @@ export default async function EditProductPage({ params }: { params: { id: string
               <p className="text-xs text-muted">Current photo. Upload a new one below to replace it.</p>
             </div>
           )}
-          <PhotoPrintAreaField />
+          <PhotoPrintAreaField existingImageUrl={product.imageUrl} />
         </div>
 
         <div>
@@ -281,7 +281,7 @@ export default async function EditProductPage({ params }: { params: { id: string
               <p className="text-xs text-muted">Current back photo and print area. Upload a new one below to replace both.</p>
             </div>
           )}
-          <PhotoPrintAreaField variant="back" />
+          <PhotoPrintAreaField variant="back" existingImageUrl={product.imageUrlBack} />
         </div>
 
         <div>