Font selection and font ligatures

## Problem

The code font is fixed. There is no way to use the font a developer already
has configured everywhere else.

Ligatures are also off everywhere. JetBrains Mono ships them, but nothing enables them.

## Proposed solution

Font picker (only for code) and ligatures toggle for it.

## Alternatives considered

Only ligatures, no font choice.

Per-surface settings (like IntelliJ's: UI font vs editor font vs terminal font). More knobs than needed; one family for all code text matches how most developers configure their tools.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start by locating the settings UI and the code-text styling or font configuration used by the IDE. Confirm how settings are persisted and where code surfaces consume the font, then verify that a code-font picker and ligatures toggle affect all code text without changing the general UI font.

Written by the indexing model from the issue text.