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/check_commands.go

@@ -26,7 +26,8 @@ type Commands struct {
 	App []string
 }
 
-// shellLangs are the fence languages whose lines are read as commands.
+// shellLangs are the fence languages read as shell sessions: their lines
+// are checked as commands and highlighted with prompts.
 var shellLangs = []string{"sh", "shell", "bash", "console"}
 
 // commandToken finds `summer <name>` and `./bin/<app> <name>` at the start