fix: add graceful UI fallback for system ringtones on iOS

[vibhutomer]

Description

This PR resolves a UX bug where iOS users are presented with a completely blank screen when navigating to the "System Ringtones" tab. Because Apple's iOS sandbox restricts third-party apps from accessing native system notification sounds, the ringtone provider correctly returns an empty list. This PR intercepts that state and provides a graceful UI fallback, vastly improving the UX for iPhone users and maintaining the app's polished feel.

Proposed Changes

• Added a GetPlatform.isIOS check inside _buildSystemRingtonesTab() in ringtone_selection_page.dart.
• Intercepted the empty list render and replaced it with a centered, user-friendly Text widget explaining the platform limitation.
• Styled the fallback message using the existing themeController to ensure perfect contrast and readability in both Light and Dark modes.

Fixes #899

Checklist

• Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing