frontend: add LoginGate, wire session into App, add logout link

frontend/src/main.tsx

@@ -1,9 +1,9 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
-import App from './App.tsx'
+import { LoginGate } from './LoginGate.tsx'
 
 createRoot(document.getElementById('root')!).render(
   <StrictMode>
-    <App />
+    <LoginGate />
   </StrictMode>,
 )