feat(backthink): 后台思考页接 WS 实时——ws.ts 处理 backthink 事件头插 store

后端每次思考落库发 ws.backthink(bus→hub→WS)，此前 ws.ts 无 case、addBackthink 是死方法。
新增 case `backthink`→parseBackthinkItem→s.addBackthink 头插；parseBackthinkItem 兼容
Go默认字段(ID/Trigger/At/Conclusion/ShouldPush/PushResult)与admin camelCase。
配合页面(行key=id)实现增量插入、不整页重渲。build 99模块绿。

frontend/web/src/lib/ws.ts

@@ -5,6 +5,7 @@ import {
   parseAdminChannelMessage,
   parseAgent,
   parseAwakenTool,
+  parseBackthinkItem,
   parseGraph,
   parsePersistedMessage,
   parseTool,
@@ -95,6 +96,14 @@ function dispatch(ev: { type: string; data: unknown }): void {
       if (msg) s.pushChannelEvent(msg);
       break;
     }
+    case "backthink": {
+      // design/12 §4 — a background-think pass result was persisted. Head-insert into
+      // the store so the admin backthink panel appends it in realtime (rows keyed by
+      // id → React reconciles incrementally, no whole-page re-render).
+      const item = parseBackthinkItem(r);
+      if (item) s.addBackthink(item);
+      break;
+    }
     case "thinking.start": {
       pendingThinking = "";
       break;