Add gradient header banner with brand mark

Header background is now a dark-ink-to-accent gradient instead of a
flat color, with the mountain/sun mark inlined next to the wordmark.
Text stays within the ink-dominant zone of the gradient so contrast
holds; dark mode gets its own ink value distinct from --bg so the
band doesn't blend into the page.

layouts/partials/header.html

@@ -1,6 +1,14 @@
 <header class="site">
   <div class="inner">
-    <a class="brand" href="/">{{ .Site.Title }}</a>
+    <a class="brand" href="/">
+      <svg class="brand-mark" viewBox="0 0 32 32" aria-hidden="true" focusable="false">
+        <circle class="brand-mark-sun" cx="17" cy="16" r="5.5"/>
+        <polygon class="brand-mark-peak" points="4,26 14,10 22,26"/>
+        <polygon class="brand-mark-peak" points="12,26 22,13 30,26"/>
+        <rect class="brand-mark-peak" x="0" y="25" width="32" height="2"/>
+      </svg>
+      <span>{{ .Site.Title }}</span>
+    </a>
     <nav aria-label="Primary">
       {{ range .Site.Menus.main }}
       <a href="{{ .URL }}"{{ if $.IsMenuCurrent "main" . }} aria-current="page"{{ end }}>{{ .Name }}</a>