feat(forum): add the reference forum widgets

Port the six forum widgets from the reference theme: posts list,
multi-tab posts, plate list (default and card), plate info, moderator
list and topic list. They join the schema widget catalog under a forum
group that stays hidden while the forum module is off, reuse the shared
forum cards, paginate through the widget REST client contract, and pull
the forum stylesheet/runtime onto any sidebar that hosts them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/admin-contract-report.json

@@ -1,5 +1,5 @@
 {
-    "generated_at": "2026-11-11T06:33:58+00:00",
+    "generated_at": "2026-11-11T07:46:58+00:00",
     "errors": [],
     "warnings": [
         "customPage tools-update section 'tools/update' not in PHP schema (ok if pure operational page)",