fix: warn user when both mobile push and email notifications are enabled #39726

[PrinceGautam2106]

This PR adds a small warning in the notifications preferences
section when user has both mobile push and email notifications
turned on at the same time, which can cause duplicate alerts.

User Experience Improvements:

• added a warning Callout in NotificationPreferencesPage
  that shows up when both mobilePushNotifications and
  emailNotifications are not set to nothing
• imported Callout from @rocket.chat/fuselage into the
  existing import line (no new import added)

Localization:

• added Mobile_push_and_email_duplicate_warning key in
  en.i18n.json for the warning message

Issue(s)

closes #2839

Steps to test or reproduce

1. go to Avatar → My Account → Notifications Preferences
2. set Mobile Push Notifications to Every Message or Every Mention
3. set Email Notifications to Every Message or Every Mention
4. warning callout should appear below the email field
5. set either one back to Nothing and warning disappears

Further comments

pretty straightforward change, just a conditional render and
one translation string. let me know if the callout should be
placed somewhere else or if the warning message needs rewording

Summary by CodeRabbit

• New Features
  • Added a warning in notification preferences that alerts users when both mobile push and email notifications are enabled simultaneously, helping them avoid receiving duplicate notification alerts.

[dwakshar]

Hi, I'd like to work on this issue. Could someone assign it to me? I'm a GSoC 2026 candidate exploring the codebase.