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/assets/theme-init.js

@@ -0,0 +1,22 @@
+// SummerCMS docs: applies the stored colour theme before first paint.
+// Loaded synchronously in <head>, before the stylesheet. "system" (the
+// default) sets neither class and lets prefers-color-scheme decide.
+(function () {
+  "use strict";
+  var root = document.documentElement;
+  var theme = "system";
+  try {
+    var stored = window.localStorage.getItem("summer-docs-theme");
+    if (stored === "light" || stored === "dark") {
+      theme = stored;
+    }
+  } catch (e) {
+    // Storage is unavailable (private mode, disabled): behave as system.
+  }
+  root.classList.add("js");
+  root.classList.remove("light", "dark");
+  if (theme !== "system") {
+    root.classList.add(theme);
+  }
+  root.setAttribute("data-theme", theme);
+})();