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

go.mod

@@ -6,6 +6,7 @@ go 1.27.0
 ignore ./admin/node_modules
 
 require (
+	github.com/alecthomas/chroma/v2 v2.27.0
 	github.com/disintegration/imaging v1.6.2
 	github.com/fsnotify/fsnotify v1.10.1
 	github.com/go-gormigrate/gormigrate/v2 v2.1.7
@@ -48,6 +49,7 @@ require (
 	github.com/containerd/platforms v0.2.1 // indirect
 	github.com/cpuguy83/dockercfg v0.3.2 // indirect
 	github.com/distribution/reference v0.6.0 // indirect
+	github.com/dlclark/regexp2/v2 v2.2.1 // indirect
 	github.com/docker/go-connections v0.7.0 // indirect
 	github.com/docker/go-units v0.5.0 // indirect
 	github.com/ebitengine/purego v0.10.1 // indirect