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/pk/components/PkHist.vue

@@ -0,0 +1,63 @@
+<script setup lang="ts">
+import { useThemeVars } from 'naive-ui'
+
+/**
+ * 全班分布：横轴做对几道（0–30，再多的堆在最右一档），一根柱一档，虚线是中间那位同学。
+ * 人少的档（不到 3 个）淡一点，一眼看到大部分人停在哪。
+ */
+const props = withDefaults(defineProps<{ values: number[]; median: number; color: string; height?: number }>(), { height: 44 })
+
+const theme = useThemeVars()
+
+const CAP = 30
+const WIDTH = 560
+
+const bars = computed(() => {
+  const counts = new Map<number, number>()
+  for (const value of props.values) {
+    const key = Math.min(value, CAP)
+    counts.set(key, (counts.get(key) ?? 0) + 1)
+  }
+  const tallest = Math.max(16, ...counts.values())
+  const width = WIDTH / (CAP + 1)
+  return {
+    width,
+    list: [...counts].map(([value, n]) => {
+      const h = Math.max(2, (props.height * n) / tallest)
+      return { value, n, x: value * width + 0.5, y: props.height - h, h }
+    }),
+    median: (Math.min(props.median, CAP) + 0.5) * width,
+  }
+})
+</script>
+
+<template>
+  <svg class="hist" :viewBox="`0 0 ${WIDTH} ${height + 12}`" role="img" aria-label="全班做对几道的分布">
+    <line :x1="0" :x2="WIDTH" :y1="height + 0.5" :y2="height + 0.5" :stroke="theme.dividerColor" />
+    <rect
+      v-for="bar in bars.list"
+      :key="bar.value"
+      :x="bar.x"
+      :y="bar.y"
+      :width="bars.width - 1"
+      :height="bar.h"
+      rx="1"
+      :fill="color"
+      :opacity="bar.n >= 3 ? 0.9 : 0.55"
+    >
+      <title>做对 {{ bar.value }}{{ bar.value === CAP ? ' 道以上' : ' 道' }}：{{ bar.n }} 人</title>
+    </rect>
+    <line :x1="bars.median" :x2="bars.median" y1="0" :y2="height" :stroke="theme.textColor2" stroke-dasharray="2 2" />
+    <text x="0" :y="height + 11" font-size="9" :fill="theme.textColor3">0</text>
+    <text :x="15 * bars.width" :y="height + 11" font-size="9" :fill="theme.textColor3" text-anchor="middle">15</text>
+    <text :x="WIDTH" :y="height + 11" font-size="9" :fill="theme.textColor3" text-anchor="end">30+</text>
+  </svg>
+</template>
+
+<style scoped>
+.hist {
+  display: block;
+  width: 100%;
+  height: auto;
+}
+</style>