Package the shared theme and page scripts as a Quarto extension #202

Found in the Quarto idiom audit.

Each site copies the same theme and script files from qwt by hand. Checked 2026-11-02: offwhite.scss is byte-identical in lds and mds, and theme-picker.html is byte-identical in mds and pds. page-navigation.html and styles.css are copied the same way and have already drifted between sites. qwt-reveal-toggle.html, tabset-code-in-tabs.html and anchor-landing.html are copies too.

Quarto's native way to share this is a custom format extension: contributes: formats: can ship SCSS in theme:, shared filters under common:, and is used as format: qwt-html. Pin versions with quarto add owner/repo@tag. Colors and fonts can also move into a shared _brand.yml.

Proposal: one Morrison-Lab/qwt-theme extension holding the files above and the two revealjs filters from #201. I did not confirm whether Dependabot can bump extensions; if it cannot, add a gha check that flags a stale pinned tag. I did not confirm that include-in-header files can be bundled in a format extension (the page does not document it), so test that first.

Effort: medium to large (one PR per site after the extension exists). Do #201 first.