feat(frontend): bootstrap React + Vite + Tailwind + Router + Layout

packages/frontend/src/components/Layout.tsx

@@ -0,0 +1,16 @@
+import { Link, Outlet } from 'react-router-dom';
+
+export function Layout() {
+  return (
+    <div className="flex h-full flex-col">
+      <header className="border-b border-slate-200 bg-white px-6 py-3 dark:border-slate-800 dark:bg-slate-900">
+        <nav className="flex gap-4 text-sm">
+          <Link to="/" className="font-semibold">Flashcards</Link>
+          <Link to="/admin">Admin</Link>
+          <Link to="/stats">Stats</Link>
+        </nav>
+      </header>
+      <main className="flex-1 overflow-auto"><Outlet /></main>
+    </div>
+  );
+}