fix(screensaver): add folder path config for screensaver folder mode

[alifakbxr]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "x" next to an item)

• Documentation update
• Bug fix
• New feature
• Other, please explain:

What changes did you make? (Give an overview)

Resolves #1560

This pull request addresses the issue where the screensaver folder mode permanently falls back to the default resources/frames/ directory due to a missing UI input and a hardcoded path.

Specifically, this PR:

1. Adds a new folder_source parameter to ScreensaverConfiguration.php (defaulting to 'screensavers').
2. Adds a new UI input field for screensaver_folder_source in the Admin Panel (lib/configsetup.inc.php).
3. Updates assets/js/screensaver.js to dynamically read from config.screensaver.folder_source instead of using a hardcoded 'screensavers' path when resolving images.
4. Adds necessary UI translation keys for the new folder configuration input in resources/lang/en.json.

Is there anything you'd like reviewers to focus on?

Please double check if there are any other specific translation files that need to be actively pushed prior to release for screensaver:screensaver_folder_source and manual:screensaver:screensaver_folder_source, or if they will be handled normally via Crowdin.

AI used to create this Pull Request?

Yes, an AI Coding Assistant was used to identify the root cause of the fallback bug and scaffold the implementation. The AI analyzed the configuration tree and Admin UI components, then mapped and updated the missing variables so users can now explicitly define a folder path for their screensaver. The changes were strictly limited to the missing screensaver configuration input and the javascript path resolution logic.