These fonts apply to markdown headings (h1-h6), so "heading" is more
accurate than "title". Renames the chosen*Font field, store action, and
the --heading-font-family CSS variable.
Keeps backward compatibility: .remanso.json still honors the legacy `t`
key (alongside the new `h`), and a saved `chosenTitleFont` in
localStorage is migrated on load so existing preferences survive.
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