Allow custom UI font selection in the desktop app

Feature request

The desktop app currently offers only two UI font options — "Anthropic Sans" and "System". Neither works well for me, and there's no way to pick a different font for the interface/chat text (no settings.json key, no theming override).

What I'd like

A way to choose a custom UI font — either a free-text font-family field (using any font installed on my system) or an expanded picker. Ideally as a separate setting from the code/monospace font, so the two can be configured independently.

Why

Readability and comfort are personal. Being locked to two fonts is a real friction point for daily use, especially for long sessions. Most editors (VS Code, etc.) let users set the UI font freely.

Environment

Claude Code desktop, Windows 11.