None of the UI sans/mono font stacks (themes/presets.ts, styles.css) carry
emoji glyphs, so on platforms whose default text font lacks them (e.g. Linux)
emoji rendered as tofu boxes in the composer and chat.
Append a color-emoji fallback — Apple Color Emoji / Segoe UI Emoji / Segoe UI
Symbol / Noto Color Emoji / the `emoji` generic — to every font stack
(SYSTEM_SANS, SYSTEM_MONO, the Courier theme, and the CSS --dt-font-* defaults).
Text still uses the primary fonts; the browser only falls back for emoji
codepoints. Custom themes build on SYSTEM_* so they inherit it automatically.
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