fix(desktop): render Nerd Font UI glyphs

[Antikythera]

What does this PR do?

Hermes Desktop can now render Nerd Font private-use glyphs in ordinary desktop UI text when a compatible Nerd Font is installed locally. This covers chat prose, code/mono surfaces, system messages, tool output, and theme-provided typography. Previously those glyphs could render as tofu boxes even though the same font worked in the embedded terminal.

Root cause

The embedded terminal and the desktop renderer use separate typography paths. terminal.font_family configures xterm.js, while chat and the rest of the renderer use the CSS families exposed through --dt-font-sans and --dt-font-mono. Those UI stacks had no Nerd Font fallback, and a custom, user, or backend theme could replace the built-in typography entirely.

Putting a full Nerd Font directly at the front of every UI stack would fix the icons but could also change normal prose/code metrics and interfere with color emoji. This PR instead declares a virtual local face, Hermes Nerd Glyphs, whose unicode-range is restricted to the Nerd Font private-use planes:

• U+E000-F8FF
• U+F0000-F2FFF

The face prefers Symbols Nerd Font Mono / Symbols Nerd Font, then accepts JetBrains Mono Nerd Font as a fallback. It is placed first in the family list so platform fonts such as Segoe UI Symbol cannot claim a private-use codepoint first, but it is ineligible for ordinary text and emoji because of the restricted range. If none of the local font names exists, Chromium skips the face and the current typography continues unchanged.

Resolved theme typography is normalized at application time, so custom, user, and backend themes cannot accidentally drop the glyph fallback. Existing entries are moved to the front and deduplicated.

Related Issue

No matching issue was found for the desktop chat/UI typography path.

Related but intentionally not duplicated:

• feat(desktop): add configurable terminal font picker #76395 configures the embedded terminal font; it does not affect renderer CSS typography.
• fix(desktop): add Nerd Font fallbacks to terminal font stacks #72153 proposes additional xterm.js terminal fallback names.
• [Enhancement] Hermes CLI / Desktop uses Unicode symbols not available in Windows 10 default fonts — displays as tofu (□) #67151 reports missing glyphs on a Windows machine without a Nerd Font installed. This PR degrades safely in that environment but does not claim to fix or close it.
• fix(desktop): replace review emoji with bundled icon #81037 separately replaces the self-improvement status's 💾 emoji with a bundled Codicon; that standard emoji is not a Nerd Font private-use glyph.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• apps/desktop/src/styles.css — declare the local-only, PUA-scoped Hermes Nerd Glyphs face and add it to the boot-time sans/mono variables.
• apps/desktop/src/themes/presets.ts — add the canonical glyph family, include it in built-in typography, and normalize external stacks without duplicates or invalid empty values.
• apps/desktop/src/themes/context.tsx — apply the normalization after built-in, user, and backend theme typography has been resolved.
• apps/desktop/src/themes/presets.test.ts — cover built-in presence, external-stack preservation, priority, deduplication, and the empty-stack edge case while retaining the existing emoji fallback contract.

How to Test

1. Install either Symbols Nerd Font Mono or JetBrainsMono Nerd Font Mono.
2. Build and launch the packaged desktop app.
3. Render private-use test glyphs in chat, for example 󰣇 󰉋 󰆍 󰄬.
4. Verify they render as icons rather than tofu boxes in prose and mono/code surfaces.
5. Verify ordinary prose/code typography and emoji remain unchanged.
6. Select a theme with custom typography and repeat the glyph check.
7. On a system without any matching Nerd Font, verify the app keeps its existing font stack and remains usable.

Automated and packaged-app validation on Bazzite Linux, KDE Plasma/Wayland, Electron 40.10.2:

Check	Result
npm run test:ui -- src/themes/presets.test.ts	13/13 passed
npm run typecheck	passed
npm run lint	0 errors; existing unrelated warnings only
git diff --check	passed
hermes desktop --build-only	packaged successfully
Packaged-app UI check	Nerd private-use glyphs rendered in chat

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched open and merged issues/PRs and separated this renderer change from terminal work in fix(desktop): add Nerd Font fallbacks to terminal font stacks #72153 and feat(desktop): add configurable terminal font picker #76395
• My PR contains only changes related to this fix (one commit, four desktop files)
• I've run pytest tests/ -q — not run; this change is isolated to the TypeScript/CSS desktop renderer and the relevant desktop checks above pass
• I've added focused tests for the theme/fallback behavior
• I've tested on my platform: Bazzite Linux, KDE Plasma/Wayland, Electron 40.10.2

Documentation & Housekeeping

• Documentation: N/A — no user-facing command, setting, or configuration key changed
• cli-config.yaml.example: N/A — no configuration keys changed
• CONTRIBUTING.md / AGENTS.md: N/A — no architecture or workflow contract changed
• Cross-platform impact considered: macOS, Windows, and Linux local font names are optional; unresolved names degrade to the existing stack
• Tool descriptions/schemas: N/A — no tool behavior changed

Screenshots / Logs

Packaged desktop UI

Manual packaged-app verification string:

󰣇 󰉋 󰆍 󰄬

Before the change these private-use codepoints could render as tofu boxes in the renderer. With a matching local Nerd Font they render as icons; normal UI typography remains selected for non-PUA text.