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>

assets/js/forum-runtime.js

@@ -1,6 +1,8 @@
 (() => {
   const config = window.jioForum || {}
-  const root = document.getElementById('forum')
+  // 论坛路由根节点是 #forum；论坛小工具放在任意侧栏时没有该节点，
+  // 此时把委托挂到 body，让 follow / apply 等小工具按钮共用同一套动作。
+  const root = document.getElementById('forum') || document.body
   if (!root) return
 
   const send = async (path, method = 'GET', data = {}, idempotencyKey = '') => {