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]/feedback-form.tsx

@@ -0,0 +1,55 @@
+"use client";
+
+import { useState, useTransition } from "react";
+import { useRouter } from "next/navigation";
+import { submitFeedback } from "./actions";
+
+export function FeedbackForm({ submissionId }: { submissionId: string }) {
+  const [text, setText] = useState("");
+  const [pending, startTransition] = useTransition();
+  const router = useRouter();
+
+  function handleSubmit(e: React.FormEvent) {
+    e.preventDefault();
+    if (!text.trim()) return;
+    startTransition(async () => {
+      await submitFeedback(submissionId, text.trim());
+      router.push("/curator/homework");
+    });
+  }
+
+  return (
+    <form onSubmit={handleSubmit} className="space-y-3">
+      <p className="text-xs font-bold uppercase tracking-widest" style={{ color: "var(--muted-foreground)" }}>
+        Написать фидбек
+      </p>
+      <textarea
+        value={text}
+        onChange={(e) => setText(e.target.value)}
+        required
+        placeholder="Напишите обратную связь студенту..."
+        style={{
+          border: "2px solid var(--border)",
+          background: "var(--background)",
+          outline: "none",
+          width: "100%",
+          padding: "0.5rem 0.75rem",
+          fontSize: "0.875rem",
+          fontFamily: "inherit",
+          resize: "vertical",
+          minHeight: "120px",
+        }}
+        onFocus={(e) => (e.currentTarget.style.borderColor = "var(--foreground)")}
+        onBlur={(e) => (e.currentTarget.style.borderColor = "var(--border)")}
+      />
+      <button
+        type="submit"
+        disabled={pending || !text.trim()}
+        className="btn-aubade btn-aubade-accent px-5 py-2 text-sm"
+        style={{ opacity: pending || !text.trim() ? 0.6 : 1 }}
+      >
+        {pending ? "Отправка..." : "Отправить фидбек"}
+      </button>
+    </form>
+  );
+}