fix(mobile/MobileChat): add aria-label to retry button for screen readers

The retry button inside the chat history error state had no accessible
label — screen reader users would encounter an unlabeled button. Added
aria-label="Retry loading chat history".

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/mobile/MobileChat.tsx

@@ -407,6 +407,7 @@ export function MobileChat({
             <div style={{ marginBottom: 8 }}>Could not load chat history.</div>
             <button
               type="button"
+              aria-label="Retry loading chat history"
               onClick={() => {
                 setLoading(true);
                 setHistoryError(null);