Add CSV import/export for students (Stage 11)

Import wizard (4 steps):
- Upload CSV with UTF-8 or Windows-1251 (iconv-lite) decoding
- Auto-detect columns: Email, Имя, Фамилия, Телефон
- Preview table with per-row status: new / update / error
- Options: auto-verify email, assign course + access days, send welcome email
- Apply: creates users with bcrypt password + Account record, grants enrollments

Export:
- GET /api/admin/export-users with course filter + encoding selection
- UTF-8 with BOM (works in all apps) or Windows-1251 (legacy Excel)
- Fields: Email, Имя, Телефон, Дата регистрации, Курсы, Прогресс

Navigation: added "Импорт / Экспорт" link to admin sidebar

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/import-export", label: "Импорт / Экспорт" },
   { href: "/admin/settings", label: "Настройки" },
 ];