feat: bundled Nerd Font symbols fallback + per-device terminal font setting

Shell prompts using Nerd Font glyphs (powerline, p10k/starship folder and
git icons) rendered as missing-glyph boxes: the built-in xterm stack has no
private-use-area symbols, and phones have no Nerd Fonts installed at all.

- Bundle Symbols Nerd Font Mono (icons-only, MIT, 1.2MB woff2) served from
  fonts/ and appended to the terminal stack before monospace — browsers fall
  back per glyph, so icons render everywhere while text stays in the text
  fonts. font-display: block + preload keep tofu out of xterm's glyph atlas.
- New per-device terminalFontFamily setting (App Settings > Terminal &
  Input > Font): prepended to the built-in stack, never a replacement, so
  the symbols fallback and final monospace always survive. Applied live on
  save (refit + echo-overlay refreshFont, mirroring setFontSize).
- Single source for both xterm surfaces: TERMINAL_FONT_DEFAULT_STACK +
  resolveTerminalFontFamily() in constants.js, unit-tested.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VjnbbZRBuvR5E3SDouwXr9

test/terminal-font.test.ts

@@ -0,0 +1,58 @@
+import { readFileSync } from 'node:fs';
+import { resolve } from 'node:path';
+import vm from 'node:vm';
+import { describe, expect, it } from 'vitest';
+
+function loadFontHelper() {
+  const context = vm.createContext({ window: {}, globalThis: {} });
+  const source = readFileSync(resolve(import.meta.dirname, '../src/web/public/constants.js'), 'utf8');
+  vm.runInContext(source, context, { filename: 'constants.js' });
+  return (
+    context.window as {
+      CodemanTerminalFont: {
+        DEFAULT_STACK: string;
+        resolve: (custom?: unknown) => string;
+      };
+    }
+  ).CodemanTerminalFont;
+}
+
+const font = loadFontHelper();
+
+describe('CodemanTerminalFont', () => {
+  it('returns the default stack for empty or missing input', () => {
+    expect(font.resolve(undefined)).toBe(font.DEFAULT_STACK);
+    expect(font.resolve('')).toBe(font.DEFAULT_STACK);
+    expect(font.resolve('   ')).toBe(font.DEFAULT_STACK);
+    expect(font.resolve(42)).toBe(font.DEFAULT_STACK);
+  });
+
+  it('keeps the symbols fallback ahead of monospace in the default stack', () => {
+    const symbolsAt = font.DEFAULT_STACK.indexOf('"Symbols Nerd Font Mono"');
+    const monoAt = font.DEFAULT_STACK.lastIndexOf('monospace');
+    expect(symbolsAt).toBeGreaterThan(-1);
+    expect(monoAt).toBeGreaterThan(symbolsAt);
+  });
+
+  it('prepends a custom family and preserves the full default stack', () => {
+    expect(font.resolve('Menlo')).toBe(`Menlo, ${font.DEFAULT_STACK}`);
+  });
+
+  it('quotes names that need quoting for CSS', () => {
+    expect(font.resolve('JetBrainsMono Nerd Font')).toBe(`"JetBrainsMono Nerd Font", ${font.DEFAULT_STACK}`);
+  });
+
+  it('normalizes already-quoted input instead of double-quoting', () => {
+    expect(font.resolve('"JetBrainsMono Nerd Font"')).toBe(`"JetBrainsMono Nerd Font", ${font.DEFAULT_STACK}`);
+    expect(font.resolve("'Iosevka Term'")).toBe(`"Iosevka Term", ${font.DEFAULT_STACK}`);
+  });
+
+  it('accepts a comma-separated list', () => {
+    expect(font.resolve('Iosevka, MesloLGS NF')).toBe(`Iosevka, "MesloLGS NF", ${font.DEFAULT_STACK}`);
+  });
+
+  it('drops generic families so they cannot shadow the symbols fallback', () => {
+    expect(font.resolve('monospace')).toBe(font.DEFAULT_STACK);
+    expect(font.resolve('Hack, monospace')).toBe(`Hack, ${font.DEFAULT_STACK}`);
+  });
+});