Fix Sonner toaster blocking button clicks on login screen

Sonner's toaster container has z-index: 999999999 with no pointer-events: none,
creating an invisible overlay that intercepts clicks. Set pointer-events: none on
the container and pointer-events: auto on individual toasts so buttons underneath
remain clickable while toasts stay interactive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -21,7 +21,12 @@ ReactDOM.createRoot(document.getElementById('root')!).render(
     <QueryClientProvider client={queryClient}>
       <BrowserRouter>
         <App />
-        <Toaster position="top-right" theme="dark" />
+        <Toaster
+          position="top-right"
+          theme="dark"
+          style={{ pointerEvents: 'none' }}
+          toastOptions={{ style: { pointerEvents: 'auto' } }}
+        />
       </BrowserRouter>
     </QueryClientProvider>
   </React.StrictMode>