Polish: homework filters, users search/popup, admin comments

Homework (/curator/homework):
- Search by student name/email
- Filter by status (pending/reviewed) and course
- Server-side pagination (20 per page) with URL params

Users (/admin/users):
- Search by name/email, filter by role
- Hover popup on each row: enrolled courses + expiry dates + email
- Pagination (20 per page) with URL params

Comments (/admin/comments):
- New admin page with all active comments
- Search by author or text content
- One-click delete (soft-delete) from the table
- Pagination (30 per page)
- Added "Комментарии" link to admin nav

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

src/components/admin/admin-nav.tsx

@@ -9,6 +9,7 @@ const links = [
   { href: "/admin/categories", label: "Категории" },
   { href: "/admin/users", label: "Пользователи" },
   { href: "/curator/homework", label: "ДЗ на проверку" },
+  { href: "/admin/comments", label: "Комментарии" },
   { href: "/admin/import-export", label: "Импорт / Экспорт" },
   { href: "/admin/settings", label: "Настройки" },
 ];