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

cmd/summer/docs.go

@@ -3,6 +3,10 @@ package main
 import (
 	"context"
 	"errors"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
 
 	"git.golem15.com/golem15/summercms/internal/docsite"
 	"git.golem15.com/golem15/summercms/modules/backpack"
@@ -79,6 +83,43 @@ func docsSyncCommand() bonfire.Command {
 	}
 }
 
+func docsServeCommand() bonfire.Command {
+	return bonfire.Command{
+		Name:        "docs:serve",
+		Description: "Build the documentation site and preview it on a local address",
+		Flags: []bonfire.Flag{
+			{Name: "root", Description: "Repository root; src= paths and modules/ resolve against it", Default: "."},
+			{Name: "src", Description: "Docs source directory (default <root>/docs)"},
+			{Name: "base-url", Description: "Base URL for site links (overrides site.yaml base_url)"},
+			{Name: "addr", Description: "Listen address; must be loopback unless --allow-remote", Default: docsite.DefaultServeAddr},
+			{Name: "allow-remote", Description: "Allow a non-loopback --addr (serves the docs on the network)", Bare: true},
+		},
+		Run: func(ctx context.Context, in bonfire.Input, out bonfire.Output) error {
+			addr, _ := in.Flag("addr")
+			if addr == "" {
+				addr = docsite.DefaultServeAddr
+			}
+			ctx, stop := signal.NotifyContext(ctx, os.Interrupt, syscall.SIGTERM)
+			defer stop()
+			return docsite.Serve(ctx, docsOptions(in), addr, flagTrue(in, "allow-remote"), outputWriter{mu: &sync.Mutex{}, out: out})
+		},
+	}
+}
+
+// outputWriter adapts bonfire.Output to io.Writer for docsite.Serve, which
+// writes from its watch goroutine too.
+type outputWriter struct {
+	mu  *sync.Mutex
+	out bonfire.Output
+}
+
+func (w outputWriter) Write(p []byte) (int, error) {
+	w.mu.Lock()
+	defer w.mu.Unlock()
+	w.out.Printf("%s", p)
+	return len(p), nil
+}
+
 func docsOptions(in bonfire.Input) docsite.Options {
 	opts := docsite.Options{Commands: docsCommands()}
 	opts.Root, _ = in.Flag("root")