Filter the Code/Terminal font picker to monospace fonts instead of rejecting non-mono picks after selection

[sudo-tmoney29]

Problem

The Code and Terminal font pickers list every installed family. Picking a proportional
font (e.g. "Bauhaus 93") closes the picker and shows an error toast:

"Bauhaus 93" isn't monospace — Code and terminal need a fixed-width font, so the current font was kept.

The app already knows which fonts are valid, but it lets the user choose an invalid one and
then refuses it. On a machine with hundreds of fonts, finding a usable one becomes trial and error.

Current behavior (code refs)

• FontFamilyPicker.tsx builds items from queryInstalledFontFamilies() and only filters by the search query.
• requireMonospace is enforced only in handlePick, via isMonospaceFamily(value), after the selection.
• isMonospaceFamily() in appearanceFonts.ts already does the detection: canvas glyph-advance
  comparison across 4 weight/style variants.

Proposal

When requireMonospace is true, apply isMonospaceFamily while building items, so only fixed-width
families appear:

const monoFamilies = useMemo(
  () => (requireMonospace ? families.filter(isMonospaceFamily) : families),
  [families, requireMonospace],
);
// build `items` from monoFamilies instead of families

Notes:

• Cost: about 32 measureText calls per family (4 variants × 8 glyphs). Run it once per enumeration
  (memoized, or cached next to installedFamiliesCache), not on every keystroke.
• Keep the handlePick guard as a backstop for edge cases such as fonts that haven't loaded yet.
• Optional: an empty-state message ("No monospace fonts match") or a small "Show all fonts" toggle,
  for users with a mono font that the detection misclassifies.

Why

It removes a dead-end interaction, reuses existing logic with no new detection code,
and makes the picker's contents match what the setting actually accepts.

Happy to send a PR if the direction is approved.

[juliusmarminge]

Note

Grok responding on behalf of Julius.

Thanks for the write-up. This tradeoff was already decided in #5656 and shipped in #15642: the Code/Terminal pickers show the full installed catalog and run isMonospaceFamily only on the selected family, so proportional picks get the toast and keep the previous font.

Catalog-wide filtering (what you propose) is what froze Appearance on large Windows/Electron font catalogs (#15576), so we intentionally moved away from it. Keeping the toast guard as a backstop is already the design.

Closing this as a duplicate of that decision. If you have a cheaper way to prefilter (e.g. metadata / worker OffscreenCanvas without main-thread catalog scans), please open a new idea with that approach.