Desktop accessibility request: low-vision friendly font scaling and UI customization

Hi team,

I’m using Goose Desktop with vision limitations, and right now the Desktop UI is hard to use comfortably.

The built-in zoom (`Ctrl++`) is useful, but it only scales the entire interface uniformly. It does not fix the readability issues caused by the current typography balance between body text, headings, inline code, lists, and code blocks.

As far as I can tell, Goose Desktop currently lacks user-facing controls for:
- font size
- typography scale
- line height / reading density
- UI font selection
- custom CSS/theme overrides

This is not just a cosmetic preference — it is an accessibility/usability issue for low-vision users.

A few possible solutions:
- font size setting in Desktop preferences
- “comfortable” / “low vision” typography presets
- adjustable line-height / density
- support for custom CSS or theme token overrides
- more granular theme customization beyond light/dark

From looking at the codebase, Goose Desktop already seems to use theme tokens for fonts and sizing, so this request may align well with the existing styling architecture.

I’d really appreciate if accessibility for low-vision users could be considered here. It would make Goose much more usable in practice.

Thanks.