fix(canvas/a11y): add accessible name to ConfirmDialog backdrop click area

WCAG 2.4.6: interactive backdrop div now has aria-label="Dismiss dialog"
and cursor-pointer so screen reader users can identify the dismiss target.
Matches the same fix applied to KeyboardShortcutsDialog in PR #299.

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

canvas/src/components/ConfirmDialog.tsx

@@ -106,7 +106,11 @@ export function ConfirmDialog({
   return createPortal(
     <div className="fixed inset-0 z-[9999] flex items-center justify-center">
       {/* Backdrop */}
-      <div className="absolute inset-0 bg-black/60 backdrop-blur-sm" onClick={onCancel} />
+      <div
+        className="absolute inset-0 bg-black/60 backdrop-blur-sm cursor-pointer"
+        onClick={onCancel}
+        aria-label="Dismiss dialog"
+      />
 
       {/* Dialog — role="dialog" + aria-modal prevent interaction with background */}
       <div