Settings → Appearance gains a Chat Font row next to Terminal Font. The value
lands in config.yaml as desktop.font_family and is layered in front of the
active theme's fontSans when the theme paints --dt-font-sans, so an empty value
is exactly the theme and a chosen family keeps the theme's CJK/emoji fallbacks.
Why: #72485 asked for OpenDyslexic in the app; #76395 only made the terminal
pane configurable, and chat/chrome typography had no user-facing knob at all
(theme presets set fontSans, imported VS Code themes carry no font opinion).
Refs #72485, #37566
40f2702b22