feat(renderer): mount Toaster in App.tsx

src/renderer/App.tsx

@@ -1,5 +1,6 @@
 import { AppShell } from './components/layout/AppShell';
 import { ModalLayer } from './components/modals/ModalLayer';
+import { Toaster } from './components/ui/sonner';
 import { useWelcomeTrigger } from './hooks/use-welcome-trigger';
 
 function App() {
@@ -8,6 +9,7 @@ function App() {
     <>
       <AppShell />
       <ModalLayer />
+      <Toaster />
     </>
   );
 }