Add appearance override setting (System / Light / Dark) (Closes #600)

[ameerhmz]

Summary

This PR resolves #600 by adding a user setting in Settings -> General to override the app's appearance mode:

• System (Default)
• Light
• Dark

Changes

1. Added AppearanceOverride enum in CorkShared module.
2. Added customAppearance default setting key in General Settings.swift.
3. Added Appearance Picker control in General Pane.swift.
4. Applied .preferredColorScheme(Defaults[.customAppearance].colorScheme) modifier to ContentView.

Mandatory Information

AI / LLM Use

Did you use any AIs / LLMs to create any part of this pull request?

• Yes
• No

If Yes — How was it used? (mandatory)

I used LLMs to help structure the AppearanceOverride enum in CorkShared and integrate the .preferredColorScheme modifier in ContentView.swift.

[buresdv]

Hello, please fill out the mandatory information that has to be included with every pull request :

Mandatory Information

AI / LLM Use

Did you use any AIs / LLMs to create any part of this pull request?

• Yes
• No

If Yes — How was it used? (mandatory)

I used LLMs to...

[ameerhmz]

Hi @buresdv! I have updated the PR description to include the mandatory AI disclosure section as requested. Thank you!