fix: build editor in Docker and restore admin panel content

The editor.js/editor.css were gitignored and never built during docker
build, causing 404s on the edit page. Add a Node multi-stage build step
to compile them. Also replace "nowt" placeholder in admin templates with
the actual main panel include.

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

templates/_types/post/admin/_oob_elements.html

@@ -18,5 +18,5 @@
 {% endblock %}
 
 {% block content %}
-nowt
+  {% include '_types/post/admin/_main_panel.html' %}
 {% endblock %}

templates/_types/post/admin/index.html

@@ -14,5 +14,5 @@
 {% endblock %}
 
 {% block content %}
-nowt
+  {% include '_types/post/admin/_main_panel.html' %}
 {% endblock %}