feat(举手与协作): 老师能在看板上直接打开学生正在写的代码，同一道题好几个人举手能「讲完了，都放下」，举手面板写着每人错在哪

学生编辑器报「在写哪道题」，看板接口带 editing；新增 peek / dismiss 两条协作消息，
求助记录带这道题最后一次提交（判完新的一次会刷新）。

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/routes/classroom.ts

@@ -17,6 +17,7 @@ import { Hono } from "hono"
 
 import { requireAuth, requireTeacher, type AppEnv } from "../auth/middleware"
 import { getLoginWindow } from "../auth/session"
+import { presenceOf } from "../collab/state"
 import { db, schema } from "../db"
 import { failure, parseBody, success } from "../http"
 import { JudgeStatus } from "../judge/status"
@@ -754,6 +755,7 @@ classroomRoutes.get("/classroom/board", requireTeacher, async (c) => {
           codeToday: codeTodayUsers.has(student.userId),
           drawnToday: drawnTodayUsers.has(student.userId),
           recentAttended: attendedByUser.get(student.userId) ?? 0,
+          editing: presenceOf(student.userId)?.problemId ?? null,
         }) satisfies ClassBoardStudent,
     ),
     recentLessons: recentDays.length,