Prevent double-submit on admin forms

Disables submit buttons after first click to prevent race conditions
that could corrupt card data via concurrent writes.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/api/src/admin.ts

@@ -93,6 +93,16 @@ export function renderAdmin(cards: BoardCard[], options: { storageMode: string;
           </section>
         </section>
       </main>
+      <script>
+        document.querySelectorAll("form").forEach(f => {
+          f.addEventListener("submit", () => {
+            f.querySelectorAll("button[type=submit]").forEach(b => {
+              b.disabled = true;
+              b.textContent = "Wait...";
+            });
+          });
+        });
+      </script>
     `
   );
 }