refactor: use backend 'Day/Night/Follow system' strings

[david-allison]

Note

Assisted-by: Claude Opus 4.8

Purpose / Description

Moves 3 difficult strings to the backend

Approach

string-array values are moved to enums, as these XML files cannot accommodate TR calls.

Then move all values to have the enum as the source of truth, and drive the preferences from them.

"Follow system" needed SentenceCase, so also handle this

How Has This Been Tested?

Visually inspected the 'Appearance' screen

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[github-actions]

Important

Maintainers: This PR contains Strings changes

1. Sync Translations before merging this PR and wait for the action to complete
2. Review and merge the auto-generated PR in order to sync all user-submitted translations
3. Sync Translations again and merge the PR so the huge automated string changes caused by merging this PR are by themselves and easy to review