Add 'approve without feedback' button for homework review

Curators/admins can mark a submission APPROVED (and complete the lesson)
without writing feedback. The student sees 'ДЗ принято ✓' and can no
longer resubmit. No email is sent on silent approval.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/actions/student-actions.ts

@@ -56,7 +56,7 @@ export async function submitHomework(
     include: { feedbacks: true },
   });
 
-  if (existing?.feedbacks && existing.feedbacks.length > 0) {
+  if ((existing?.feedbacks && existing.feedbacks.length > 0) || existing?.status === "APPROVED") {
     throw new Error("Работа уже проверена");
   }