test(desktop): assert every theme typography carries an emoji font (#40364)

Regression guard for the emoji-fallback fix: checks DEFAULT_TYPOGRAPHY and every
defined builtin-theme fontSans/fontMono stack contains a color-emoji font.

apps/desktop/src/themes/presets.test.ts

@@ -0,0 +1,33 @@
+import { describe, expect, it } from 'vitest'
+
+import { BUILTIN_THEME_LIST, DEFAULT_TYPOGRAPHY, EMOJI_FALLBACK } from './presets'
+
+// #40364: none of the UI text/mono fonts carry emoji glyphs, so every font
+// stack must end with a color-emoji fallback or emoji render as tofu on
+// platforms whose default font lacks them (e.g. Linux).
+describe('theme typography emoji fallback (#40364)', () => {
+  const stacks: Array<[string, string]> = [
+    ['DEFAULT_TYPOGRAPHY.fontSans', DEFAULT_TYPOGRAPHY.fontSans],
+    ['DEFAULT_TYPOGRAPHY.fontMono', DEFAULT_TYPOGRAPHY.fontMono],
+    // A theme may override only fontMono (fontSans then falls back to the
+    // default, which already carries the emoji stack), so skip undefined.
+    ...BUILTIN_THEME_LIST.flatMap(theme =>
+      (
+        [
+          [`${theme.name}.fontSans`, theme.typography?.fontSans],
+          [`${theme.name}.fontMono`, theme.typography?.fontMono]
+        ] as Array<[string, string | undefined]>
+      ).filter((entry): entry is [string, string] => typeof entry[1] === 'string')
+    )
+  ]
+
+  it.each(stacks)('%s includes a color-emoji font', (_label, stack) => {
+    expect(stack).toMatch(/Apple Color Emoji|Segoe UI Emoji|Noto Color Emoji|(^|,\s*)emoji\b/)
+  })
+
+  it('EMOJI_FALLBACK lists the major platform emoji fonts', () => {
+    expect(EMOJI_FALLBACK).toContain('Apple Color Emoji')
+    expect(EMOJI_FALLBACK).toContain('Segoe UI Emoji')
+    expect(EMOJI_FALLBACK).toContain('Noto Color Emoji')
+  })
+})