Add in-app confirmation dialog replacing window.confirm

src/app/App.tsx

@@ -1,3 +1,4 @@
+import { ConfirmDialog } from './components/ConfirmDialog';
 import { LivePreview } from './components/LivePreview';
 import { SnippetLibrary } from './components/SnippetLibrary';
 import { SpecEditor } from './components/SpecEditor';
@@ -33,6 +34,10 @@ export function App() {
           <LivePreview />
         </section>
       </main>
+
+      {/* Global confirmation layer — sits above the (future) feature-modal
+          shell so a discard-changes prompt can appear over an open modal. */}
+      <ConfirmDialog />
     </div>
   );
 }