appearance persistence and accessibility improvements

[Zaingoya65]

• Added theme_preference column to users table and updated User model.
• Implemented quick-access theme toggle in the header.
• Added "Settings" link to the sidebar for better discoverability.
• Fixed layout nesting issues in header and sidebar.
• Includes comprehensive feature tests (AppearanceUpdateTest.php).

[krekas]

Hi, everything looks good but we need you to make some adjustment:

1. There's no need for the notification after the appearance is changed, especially when the message is appearance-updated

2. The added dropdown doesn't have active state

3. I don't think adding settings link to the navigation is necessary. Usually it's under the user, let's remove it.

[Zaingoya65]

I have updated the PR according to your feedback, I removed the notification, added active states to the theme toggle, and removed the settings link from the sidebar.

Ready for another look!

[krekas]

Didn't see at first. Remove the composer.lock from the commit

[Zaingoya65]

Thanks for catching that! I have removed composer.lock from the PR. It was accidentally included in one of my previous commits.
Everything is now updated according to your feedback

[PovilasKorop]

@Zaingoya65 thank you for your PR and the fixes, I really appreciate it.