Disable Submit for Approval button when admin is editing design

Added isAdminEditing prop to DesignCanvas. When true, the Submit for Approval
button is disabled (grayed out) since the admin should use the Send to Customer
for Review button instead. This is a safer approach than hiding the button.

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

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

@@ -110,7 +110,7 @@ export default function EditDesignPage({ params }: { params: { id: string } }) {
         </div>
 
         {/* Full Design Canvas */}
-        <DesignCanvas product={design.product} />
+        <DesignCanvas product={design.product} isAdminEditing={true} />
       </div>
     </div>
   );