feat: support all monospaced fonts installed on the local system

[nexxai]

Problem

I wanted to use a monospaced font that wasn't one of the 4 default included ones (specifically JetBrains Mono) but was obviously unable to as the list was hardcoded.

Solution

I replaced the hardcoded font option flow with a shared runtime resolver that discovers installed monospaced font families and uses them in both settings UI and runtime theme font resolution. The resolver keeps System Mono as a safe default and fallback when a saved font is unavailable. Theme storage remains string-based and backward-compatible, and docs/tests were updated to cover dynamic listing and fallback behavior.

This is my first contribution to this repo so if I'm missing anything, please let me know.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[nexxai]

I have read the CLA Document and I hereby sign the CLA.

[datlechin]

Nice PR, but have few things to fix:

• isRecommended and displayName seem like definded but no read anywhere
• availableMonospacedFamilies() make it static let instead

[nexxai]

Sorry about the extra properties. I had been trying out a couple different features I've been thinking about and forgot to git add -p here. I've removed them and switched the availableMonospacedFamilies to a static let property.

[datlechin]

Hi @nexxai, thanks for the quick update. Also, please update the tests as well. Currently, the test still has parentheses around EditorFontResolver.availableMonospacedFamilies().

[nexxai]

Yeah, I guess that would be helpful to include in the commit too.