Forum integration: Flarum extension, local test stack, embedded comments

- forum/extension: oc:import command (archived comments -> one thread per
  machine) and embed styling
- forum/docker: local Flarum 2 test stack
- site: optional params.forum.url adds a Forum menu entry and shows the
  machine thread in the Comments tab

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDRjtmAiNpo5mK5N3d7QwZ

layouts/baseof.html

@@ -17,6 +17,7 @@
       {{ range site.Menus.main }}
       <a href="{{ .URL }}"{{ if or ($current.IsMenuCurrent .Menu .) ($current.HasMenuCurrent .Menu .) (eq $current.Section .Page.Section) }} class="active"{{ end }}>{{ .Name }}</a>
       {{ end }}
+      {{ with site.Params.forum.url }}<a href="{{ . }}">Forum</a>{{ end }}
     </nav>
     <form class="search" role="search" action="{{ "search/" | relURL }}" method="get">
       <input type="search" name="q" placeholder="Search…" aria-label="Search the site">