Add an option to customize the app UI font

### Feature summary

Currently, there's an option to change the terminal font. It would be nice if there were also an option to change the app's UI font.

### What problem are you trying to solve?

It improves accessibility by letting users choose a font that's easier for them to read. It also lets users customize the app's appearance and makes the experience more consistent, since the terminal font can already be changed.

### Proposed solution

Add an option in the app's settings to let users choose the UI font, similar to the existing option for changing the terminal font.

### Workflow impact

Users who want better readability, accessibility, or a more personalized experience would benefit from this. It improves the app experience without changing the existing workflow, since it's an optional customization.

### Installation context

_No response_

### Additional context

_No response_

The issue names no files or tests. Start by locating the existing terminal-font setting in the app's settings, then trace how its option is stored and applied; done means users can choose a separate UI font in settings and see it applied consistently without changing the terminal font option.

索引モデルが issue の本文から書いたものです。