Add error handling and storage safety checks to admin

Show inline error banners when card save/delete fails instead of
crashing. Prevent writes to local filesystem on Vercel where it would
silently fail by validating BLOB_READ_WRITE_TOKEN presence.

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

apps/api/src/admin.ts

@@ -29,7 +29,7 @@ export function renderLogin(message = "") {
   );
 }
 
-export function renderAdmin(cards: BoardCard[], options: { storageMode: string }) {
+export function renderAdmin(cards: BoardCard[], options: { storageMode: string; message?: string }) {
   const sorted = [...cards].sort((a, b) => Date.parse(b.updatedAt) - Date.parse(a.updatedAt));
 
   return pageShell(
@@ -52,6 +52,12 @@ export function renderAdmin(cards: BoardCard[], options: { storageMode: string }
           </aside>
         </header>
 
+        ${
+          options.message
+            ? `<section class="noticeBanner">${escapeHtml(options.message)}</section>`
+            : ""
+        }
+
         <section class="workbench">
           <form class="editor" method="post" action="/admin/cards" enctype="multipart/form-data">
             <p class="stamp">New card</p>
@@ -201,7 +207,8 @@ textarea {
 .mast,
 .editor,
 .cardEditor,
-.empty {
+.empty,
+.noticeBanner {
   border: 2px solid var(--color-line);
   background: var(--color-panel);
   box-shadow: var(--shadow-hard);
@@ -283,10 +290,18 @@ h1 {
 
 .editor,
 .cardEditor,
-.empty {
+.empty,
+.noticeBanner {
   padding: 18px;
 }
 
+.noticeBanner {
+  margin-top: 28px;
+  color: var(--color-ink);
+  font-family: var(--font-mono);
+  overflow-wrap: anywhere;
+}
+
 .editor {
   position: sticky;
   top: 18px;