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

forum/extension/less/embed.less

@@ -0,0 +1,9 @@
+// Embedded thread, as shown in the Comments tab of the museum pages: full width, on the
+// light panel colour of the static site (--panel in its assets/css/main.css).
+@oc-accent: #9a3412;
+@oc-panel: #fffdf7;
+
+body, .App { background: @oc-panel; }
+.Page-content { max-width: none; }
+.Post-body a, .PostUser-name a, .Button--link { color: @oc-accent; }
+.Button--primary { background: @oc-accent; }