feat: wire homepage builder layout system and sidebar control

- base.njk: skip hardcoded sidebar when homepage builder is active
- homepage-builder.njk: implement 3 layouts (single-column, two-column,
  full-width-hero) with CSS grid and sidebar rendering
- homepage-section.njk: section type dispatcher (extracted for reuse)
- homepage-sidebar.njk: maps widget types to existing widget partials

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

_includes/layouts/base.njk

@@ -270,7 +270,7 @@
   </header>
 
   <main class="container py-8" data-pagefind-body>
-    {% if withSidebar %}
+    {% if withSidebar and not (homepageConfig and homepageConfig.sections) %}
     <div class="layout-with-sidebar">
       <div class="main-content">
         {{ content | safe }}
@@ -279,6 +279,9 @@
         {% include "components/sidebar.njk" %}
       </aside>
     </div>
+    {% elif withSidebar and homepageConfig and homepageConfig.sections %}
+    {# Homepage builder controls its own layout and sidebar #}
+    {{ content | safe }}
     {% elif withBlogSidebar %}
     <div class="layout-with-sidebar">
       <div class="main-content">