refactor(课堂任务): 「课堂看板」改名「课堂任务」，地址换成 /task；班级 PK 换成 /pk

- 界面上的「课堂看板」「课堂作业」统一叫「课堂任务」，顶栏、老师首页、后台菜单一起改
- 前端 oj/classroom → oj/task、oj/class → oj/pk，两个名字不再只差几个字母却毫不相干；旧地址不留跳转
- 后端 routes/classroom.ts 拆开：课堂任务的接口进 routes/task.ts（/task/board|lesson|today），
  首页的「上次来」「班里在做」进 routes/home.ts；共用的取题口径挪进 services/lesson.ts
- 契约 classroom.ts 同样拆成 task.ts / home.ts，ClassBoard* 改叫 TaskBoard*

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

apps/web/src/oj/rank/components/ClassBattle.vue

@@ -55,7 +55,7 @@ function shortLabel(className: string) {
   <div class="battle" :class="{ bare }">
     <div class="title">
       <b v-if="!bare">班级对抗</b><span class="muted">人均做对</span>
-      <router-link v-if="pk" :to="{ path: '/class', query: mine ? { classes: mine } : {} }" class="pk">班级 PK ›</router-link>
+      <router-link v-if="pk" :to="{ path: '/pk', query: mine ? { classes: mine } : {} }" class="pk">班级 PK ›</router-link>
     </div>
     <div v-for="item in shown" :key="item.className" class="row" :class="{ mine: item.className === mine }">
       <span class="rank">{{ item.rank }}</span>

apps/web/src/oj/rank/components/ClassDetailDrawer.vue

@@ -213,7 +213,7 @@ function go(path: string, query: Record<string, string>) {
             <Markdown v-if="report" :content="report" />
           </section>
 
-          <a v-if="pk" href="#" class="pk" @click.prevent="go('/class', { classes: className! })">和别的班比 → 班级 PK ›</a>
+          <a v-if="pk" href="#" class="pk" @click.prevent="go('/pk', { classes: className! })">和别的班比 → 班级 PK ›</a>
         </div>
         <div v-else-if="!loading" class="muted">这个班的数据没取到</div>
       </n-spin>

apps/web/src/oj/rank/list.vue

@@ -441,7 +441,7 @@ async function restore(userId: number) {
                 secondary
                 @click="
                   router.push({
-                    path: '/class',
+                    path: '/pk',
                     query: classContext ? { classes: classContext } : {},
                   })
                 "