Add quiz feature: student UI, admin editor, lesson page integration

- QuizSection component: shows questions as text inputs, read-only after submission
- QuizEditor component: admin CRUD for quiz questions with type selector
- saveQuiz/deleteQuiz server actions for admin
- submitQuizAttempt server action: idempotent, auto-marks lesson complete
- Student lesson page: renders QuizSection, updates complete button logic
- Admin lesson page: renders QuizEditor below homework section

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

src/lib/actions/quiz-actions.ts

@@ -0,0 +1,53 @@
+"use server";
+
+import { prisma } from "@/lib/prisma";
+import { auth } from "@/lib/auth";
+import { headers } from "next/headers";
+import { revalidatePath } from "next/cache";
+
+async function requireAdmin() {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session || session.user.role !== "admin") throw new Error("Forbidden");
+}
+
+export async function saveQuiz(
+  lessonId: string,
+  questions: { text: string; type: "TEXT" | "SINGLE" | "MULTIPLE"; order: number }[]
+) {
+  await requireAdmin();
+
+  const existing = await prisma.quiz.findUnique({ where: { lessonId } });
+
+  if (existing) {
+    await prisma.quizQuestion.deleteMany({ where: { quizId: existing.id } });
+    await prisma.quizQuestion.createMany({
+      data: questions.map((q) => ({
+        quizId: existing.id,
+        text: q.text,
+        type: q.type,
+        order: q.order,
+      })),
+    });
+  } else {
+    await prisma.quiz.create({
+      data: {
+        lessonId,
+        questions: {
+          create: questions.map((q) => ({
+            text: q.text,
+            type: q.type,
+            order: q.order,
+          })),
+        },
+      },
+    });
+  }
+
+  revalidatePath(`/admin`);
+}
+
+export async function deleteQuiz(lessonId: string) {
+  await requireAdmin();
+  await prisma.quiz.delete({ where: { lessonId } });
+  revalidatePath(`/admin`);
+}