Add notification toasts and surface persistence failures instead of failing silently

src/app/App.tsx

@@ -4,6 +4,7 @@ import { ResizeHandle } from './components/ResizeHandle';
 import { SnippetLibrary } from './components/SnippetLibrary';
 import { SpecEditor } from './components/SpecEditor';
 import { ThemeToggle } from './components/ThemeToggle';
+import { Toaster } from './components/Toaster';
 import { usePanesStore } from './stores/PanesStore';
 import styles from './App.module.css';
 
@@ -50,6 +51,10 @@ export function App() {
       {/* Global confirmation layer — sits above the (future) feature-modal
           shell so a discard-changes prompt can appear over an open modal. */}
       <ConfirmDialog />
+
+      {/* Non-blocking notifications (failed saves, etc.) — top-right toasts,
+          layered above the confirm backdrop so a failure stays visible. */}
+      <Toaster />
     </div>
   );
 }