feat(site): a header on every page, and navigation for the docs (phase 5) (#68)

Every page gets the same header: the calculator, the docs, the example
report, and GitHub, with a theme toggle. The doc pages get a grouped docs
sidebar, an "On this page" contents list that follows the section in
view, a link on every h2 and h3 that copies itself, previous and next
links, an "Edit on GitHub" link, and copy buttons on code blocks.

All of the navigation is HTML written by build_site.py, so it works with
scripts off and check_site_links.mjs verifies every link it adds. The
renderer now also returns each doc's headings and per-section text (the
search index in the next phase reads the latter), emits the heading
links itself rather than through raw HTML, and turns table alignment
into classes so no page carries an inline style.

site/site.js adds only conveniences. theme.js applies a remembered
theme before paint, so a dark choice does not flash light.

Form borders and the histogram's bars now meet 3:1 contrast in both
themes (part of #53).

Co-authored-by: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: TMHSDigital

CHANGELOG.md

@@ -35,6 +35,14 @@ different event from one that moved because it was wrong.
   carries a canonical URL and Open Graph and Twitter card tags; the card image
   (`site/og.png`) is rendered from `scripts/og_image.html`. Missing paths get a
   404 page that links back.
+- The site has a header on every page (calculator, docs, example report,
+  GitHub), and the doc pages have a grouped docs sidebar, an "On this page"
+  contents list that follows the section in view, a link on every h2 and h3
+  that copies itself, previous and next links, an "Edit on GitHub" link, and
+  copy buttons on code blocks. All of the navigation is HTML written at build
+  time, so it works with scripts off; `site/site.js` adds only the conveniences
+  and a light, dark, or automatic theme that is remembered between visits.
+  Form borders and the histogram's bars now meet 3:1 contrast in both themes.
 
 ### Changed