fix(docs-toolkit): restore sidebar scrolling in preview:html output

FR-2768 made .doc-sidebar a fixed-height, overflow:hidden flex column
and delegated scrolling to an inner .doc-sidebar__scroll element in
styles-web.ts. website-builder.ts (build:web / serve:web) emits that
wrapper, but the legacy single-page preview builder (html-builder-web.ts,
used by preview:html / preview:html:ko) was never updated, so the nav
overflowed a clipped container with no scrollport and the sidebar could
not scroll.

Wrap the nav list in .doc-sidebar__scroll to mirror website-builder.ts,
restoring sidebar scrolling in the HTML preview server.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: agatha197

packages/backend.ai-docs-toolkit/src/html-builder-web.ts

@@ -44,15 +44,24 @@ function buildSidebarHtml(
     })
     .join('\n');
 
+  // FR-2768 made .doc-sidebar a fixed-height, overflow:hidden flex column
+  // and delegated scrolling to an inner .doc-sidebar__scroll element (see
+  // styles-web.ts). website-builder.ts emits that wrapper; this legacy
+  // single-page preview builder must too, otherwise the nav overflows a
+  // clipped container with no scrollport and the sidebar can't scroll.
+  // .doc-sidebar-header is display:none in Phase 2, so leaving it outside
+  // the scrollport mirrors the pinned version block in website-builder.ts.
   return `
 <aside class="doc-sidebar">
   <div class="doc-sidebar-header">
     <h2>${metadata.title}</h2>
     <div class="doc-meta">${metadata.version} &middot; ${langLabel}</div>
   </div>
-  <ul class="doc-sidebar-nav">
-    ${navItems}
-  </ul>
+  <div class="doc-sidebar__scroll">
+    <ul class="doc-sidebar-nav">
+      ${navItems}
+    </ul>
+  </div>
 </aside>`;
 }