Add homework system (admin, student, curator)

Admin:
- HomeworkEditor in lesson page: create/update/delete assignment description

Student:
- HomeworkSection in lesson page: view assignment, submit text + files
- Resubmission allowed until curator gives feedback
- Shows feedback from curator with date and name

Curator:
- New layout with Second Brain dark sidebar (replaces green theme)
- /curator/dashboard: stats cards (pending, total, reviewed this week)
- /curator/homework: list of all submissions, pending highlighted
- /curator/homework/[id]: review submission, write feedback, redirect after send

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

src/app/curator/homework/[submissionId]/actions.ts

@@ -0,0 +1,20 @@
+"use server";
+
+import { prisma } from "@/lib/prisma";
+import { auth } from "@/lib/auth";
+import { headers } from "next/headers";
+import { revalidatePath } from "next/cache";
+
+export async function submitFeedback(submissionId: string, text: string) {
+  const session = await auth.api.getSession({ headers: await headers() });
+  if (!session || (session.user.role !== "curator" && session.user.role !== "admin")) {
+    throw new Error("Forbidden");
+  }
+
+  await prisma.homeworkFeedback.create({
+    data: { submissionId, curatorId: session.user.id, text },
+  });
+
+  revalidatePath("/curator/homework");
+  revalidatePath(`/curator/homework/${submissionId}`);
+}