feat: make native title bar a toggle

[eleboucher]

Description

Adds a desktop setting to switch between Sable’s custom title bar and native window decorations.

• Persists the selected title bar mode.
• Applies it at startup and while running.
• Keeps connection status in the correct location.
• Fixes black gaps when resizing the Linux CEF window.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[hazre]

Why? I intentionally made a custom titlebar on windows, so it looks nice and we can have extra functionality, like loading indicator and etc.

also could please follow the PR format unless the title is self explanatory for small changes.

[7w1]

Getting rid of it on mac might makes sense, on windows imo its better to have it so we can theme it. On linux, idk. People's setups vary just like... add a toggle or something I guess.

[hazre]

lgtm