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/machine-tabs/single.html

@@ -33,6 +33,16 @@
       {{ end }}
     </div>
 
+  {{ else if and (eq $tab "comments") $p.forum_thread site.Params.forum.url }}
+    {{/* live comments: the machine's forum thread, shown in a frame sized to its content */}}
+    {{ $thread := printf "%s/embed/%d" (strings.TrimSuffix "/" site.Params.forum.url) (int $p.forum_thread) }}
+    <p class="note">The first comments were left by visitors of the original site. Log in to the forum to add yours. <a href="{{ replace $thread "/embed/" "/d/" }}">Open this discussion in the forum</a>.</p>
+    <iframe class="forum-thread" id="forum-thread" src="{{ $thread }}" title="Comments on {{ $m.Title }}" loading="lazy"></iframe>
+    {{ with resources.Get "js/iframeResizer.min.js" | fingerprint }}
+    <script src="{{ .RelPermalink }}"></script>
+    <script>iFrameResize({ checkOrigin: false, heightCalculationMethod: "lowestElement" }, "#forum-thread");</script>
+    {{ end }}
+
   {{ else if eq $tab "comments" }}
     <p class="note">Comments left by visitors of the original site. New comments are closed.</p>
     <ol class="comments">