feat: neutralize theme for fresh deployments

Strip personal data from templates so the theme ships clean for any
deployer. Collection pages now use generatePageOnEmptyData so empty
post types show encouraging placeholders instead of 404s. Navigation
is conditional on enabled post types and installed plugins. Sidebar
widgets split into individual components with plugin-aware visibility.
Slashes page explains required plugins for root-level page creation.

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

photos.njk

@@ -6,6 +6,7 @@ pagination:
   data: collections.photos
   size: 20
   alias: paginatedPhotos
+  generatePageOnEmptyData: true
 permalink: "photos/{% if pagination.pageNumber > 0 %}page/{{ pagination.pageNumber + 1 }}/{% endif %}"
 ---
 <div class="h-feed">
@@ -92,6 +93,7 @@ permalink: "photos/{% if pagination.pageNumber > 0 %}page/{{ pagination.pageNumb
   {% endif %}
 
   {% else %}
-  <p class="text-surface-600 dark:text-surface-400">No photos yet.</p>
+  {% set postType = "photo" %}
+  {% include "components/empty-collection.njk" %}
   {% endif %}
 </div>