Add a default-off "Show all fonts" escape hatch to the font pickers

[MHohlios]

Follow-up to the installed-fonts filter, as discussed: a default-off escape hatch for monospace fonts that is_monospace() misses (unusual naming, missing OS/2 flags).

• Each font picker carries a compact "Show all fonts" checkbox under its select. Both drive one shared pref (the pickers feed from the same list); tooltip clarifies the scope. Off by default, persisted, part of Reset to defaults.
• When on, availableMonoFontsAsync sources the system: extras from the full installed family catalog instead of the monospace subset. The curated installed-only filter, dedup against curated stacks, and the Bundled/Installed optgroups are unchanged.
• Flipping it re-merges from the cached enumeration (no re-scan). A selection made while on stays visible and re-selectable after turning it off (existing withSelectedFonts rescue).
• Tests: pref default/persistence/reset, plus mocked-IPC coverage that showAll surfaces a non-monospace family and default mode hides it.

[simion]

Merged, thanks.

One follow-up on main (d03c449): the "Show all fonts" row is a <label> now instead of a div with its own onClick. A label forwards text clicks to the checkbox button natively, so the stop-propagation workaround and its comment go away. Same pattern as the sandbox checkbox rows in RepositorySection.