feat(11.1-02): add the documentation theme, chroma highlighting and docs:serve

- WinterCMS-style shell: header with search and theme toggle, grouped
  sidebar, on-page TOC, pager, page actions, callouts, heading
  permalinks, footer and a 404 page
- fenced code highlighted at build time by chroma/v2 into tok-* classes,
  with a copy button; no inline script, style or handler
- vendored DM Sans/DM Mono fonts and Lucide icons with their licences
- client-side search over search-index.json built with textContent only
- summer docs:serve builds into a temp dir, serves on loopback by default,
  returns 404.html with status 404 and rebuilds on change

internal/docsite/theme/templates/header.html

@@ -0,0 +1,9 @@
+{{define "header" -}}
+<header class="site-header">
+<button class="menu-button icon-button" type="button" aria-label="Open navigation" aria-expanded="false" aria-controls="sidebar">{{template "icon-menu"}}{{template "icon-x"}}</button>
+<a class="wordmark" href="{{.HomeURL}}" aria-label="SummerCMS documentation home"><span class="wordmark-sun">{{template "icon-sun"}}</span><span class="wordmark-text">Summer<span class="wordmark-cms">CMS</span></span></a>
+<div class="header-spacer"></div>
+<button class="search-trigger" type="button" aria-label="Search docs" aria-haspopup="dialog" aria-controls="search">{{template "icon-search"}}<span class="search-trigger-label">Search docs</span><kbd>Ctrl K</kbd></button>
+<button class="theme-toggle" type="button" data-theme="system" aria-label="Color theme: System. Switch to Light." title="Color theme: System. Switch to Light.">{{template "icon-monitor"}}{{template "icon-sun"}}{{template "icon-moon"}}</button>
+</header>
+{{- end}}