feat: language selector on user creation, pass to welcome email

Adds preferred_language field to Users, language picker (EN/AR) in
create/edit user form, persists to NocoDB, and passes it to the
welcome notification so new users receive emails in their language.

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

client/src/i18n/ar.json

@@ -570,6 +570,7 @@
   "users.passwordMismatch": "كلمات المرور غير متطابقة",
   "users.passwordRequired": "كلمة المرور مطلوبة للمستخدمين الجدد",
   "users.saveFailed": "فشل في حفظ المستخدم",
+  "users.preferredLanguage": "اللغة المفضلة",
   "users.deleteFailed": "فشل في حذف المستخدم",
 
   "settings.saveFailed": "فشل في الحفظ",