The mark and wordmark were already a home link on the Jinja pages (homepage,
about, climate, glossary, privacy), but the five static tool pages — calendar,
day, compare, legend, alerts — built their header separately and left the brand
as a bare <h1>, so clicking it did nothing.
Wrap the lockup in `<a href="./">` there, matching the relative form the nav
already uses: it resolves to the app root whether the app is served at the
domain root or under a base path.
The link wraps the whole lockup, so the mark is clickable too, not just the
word. Its colour/decoration moves from `.site-name a` up to the shared
`.brand h1 a, .brand .site-name a` rule so it covers both the static pages'
<h1> form and the homepage's <p>, with an accent hover so the affordance is
visible.
Adds a parametrized test over every page: the two header implementations are
easy to change independently, so this asserts each one carries a home link that
wraps the mark.
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