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>

package.json

@@ -35,6 +35,7 @@
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "gray-matter": "^4.0.3",
+    "iconv-lite": "^0.7.2",
     "lucide-react": "^1.7.0",
     "next": "16.2.2",
     "next-themes": "^0.4.6",