feat(frontend): admin lesson tree CRUD

packages/frontend/src/pages/Admin.tsx

@@ -0,0 +1,29 @@
+import { useEffect, useState } from 'react';
+import { lessonsApi } from '../api/lessons.js';
+import { useLessons } from '../stores/lessonsStore.js';
+import { LessonTree } from '../components/LessonTree.js';
+
+export function AdminPage() {
+  const { tree, refresh, loading } = useLessons();
+  const [newRoot, setNewRoot] = useState('');
+
+  useEffect(() => { refresh(); }, [refresh]);
+
+  async function addRoot() {
+    if (!newRoot.trim()) return;
+    await lessonsApi.create({ name: newRoot.trim(), parentId: null });
+    setNewRoot('');
+    await refresh();
+  }
+
+  return (
+    <div className="mx-auto max-w-3xl p-6">
+      <h1 className="mb-4 text-2xl font-semibold">Lessen beheer</h1>
+      <div className="mb-4 flex gap-2">
+        <input className="flex-1 rounded border px-3 py-2 dark:bg-slate-900" placeholder="Nieuwe wortel-les..." value={newRoot} onChange={(e) => setNewRoot(e.target.value)} />
+        <button className="rounded bg-blue-600 px-4 py-2 text-white" onClick={addRoot}>Toevoegen</button>
+      </div>
+      {loading ? <p>Laden...</p> : <LessonTree nodes={tree} />}
+    </div>
+  );
+}