feat(settings): add required field validation for Jellyfin API settings
- Make Jellyfin Server URL, API Key, and User ID required fields
- Add asterisk (*) indicators to required field labels
- Implement client-side validation with error toast notifications
- Add visual "* Required fields" indicator in card description
- Include help link to setup documentation for User ID field
- Prevent form submission when required fields are empty
Fixes configuration issues by ensuring essential Jellyfin connection
parameters are always provided before saving settings.