Make the brand lockup a home link on every page (#182)

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.

tests/test_content.py

@@ -268,3 +268,24 @@ def test_search_verification_meta(client, monkeypatch):
     for b in (seo, home):
         assert '<meta name="google-site-verification" content="gtok123">' in b
         assert '<meta name="msvalidate.01" content="btok456">' in b
+
+
+# The brand lockup is the site-wide "go home" affordance. The static tool pages
+# and the Jinja pages build their headers separately, so this is easy to add to
+# one and forget on the other — assert every page carries it.
+BRAND_PAGES = ["/", "/calendar", "/day", "/compare", "/legend", "/alerts",
+               "/about", "/privacy", "/climate", "/glossary"]
+
+
+@pytest.mark.parametrize("path", BRAND_PAGES)
+def test_brand_lockup_links_home(client, path):
+    html = client.get(f"{B}{path}").text
+    brand = html.split('<div class="brand">', 1)[1].split("</header>", 1)[0]
+    head = brand.split("</h1>")[0] if "<h1" in brand else brand.split("</p>")[0]
+    # The link must wrap the whole lockup, so clicking the mark works too, not
+    # just the word.
+    assert "<a href=" in head, f"{path}: brand is not a link"
+    href = head.split('<a href="', 1)[1].split('"', 1)[0]
+    assert href in ("./", f"{B}/"), f"{path}: brand links to {href!r}, not home"
+    assert '<span class="logo">' in head.split("<a href=", 1)[1], \
+        f"{path}: the mark sits outside the home link"