feat: add frontend auth shell

frontend/src/app/App.tsx

@@ -1,7 +1,37 @@
 import { AppShell } from './layout/AppShell'
 import { Router } from './router'
+import { AuthProvider, useAuth } from '@/features/auth/AuthProvider'
+import { LoginPage } from '@/features/auth/LoginPage'
+import { Skeleton } from '@/components/ui/skeleton'
 
 export function App() {
+  return (
+    <AuthProvider>
+      <AppContent />
+    </AuthProvider>
+  )
+}
+
+function AppContent() {
+  const { authenticated, loading } = useAuth()
+
+  if (loading) {
+    return (
+      <main className="flex min-h-screen items-center justify-center bg-background px-5">
+        <div className="grid w-full max-w-[420px] gap-3">
+          <Skeleton className="h-9 w-44" />
+          <Skeleton className="h-4 w-full" />
+          <Skeleton className="h-4 w-3/4" />
+          <Skeleton className="mt-4 h-48 w-full" />
+        </div>
+      </main>
+    )
+  }
+
+  if (!authenticated) {
+    return <LoginPage />
+  }
+
   return (
     <AppShell>
       <Router />