fix(a11y): add role=alert to MemoryInspectorPanel error banner (#901)

The error banner div introduced in the MemoryInspectorPanel (PR #892)
was missing role="alert", regressing the a11y standard established in
PR #877 / issue #830. Screen readers now announce the error immediately
on render.

Closes #901

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

canvas/src/components/MemoryInspectorPanel.tsx

@@ -188,7 +188,7 @@ export function MemoryInspectorPanel({ workspaceId }: Props) {
 
       {/* Error banner */}
       {error && (
-        <div className="mx-4 mt-3 px-3 py-2 bg-red-950/30 border border-red-800/40 rounded text-xs text-red-400">
+        <div role="alert" className="mx-4 mt-3 px-3 py-2 bg-red-950/30 border border-red-800/40 rounded text-xs text-red-400">
           {error}
         </div>
       )}