feat(自学): 重做学生端页面与教师端学情概览

学生端：目录 + 居中限宽正文 + 可收起的示例代码栏；目录改成三态圆点，
顶部加总进度；上一课/下一课栏桌面端固定在底部。
教师端：汇总卡片、学生状态标签与筛选（未开始/7 天没学/只读不练等）、
最后学习补「N 天前」、按练习表加「没人一次对/多数人卡住」提示。

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/src/oj/learn/components/LearnSummary.vue

@@ -0,0 +1,55 @@
+<script setup lang="ts">
+import { TUTORIAL_READ_SECONDS } from "@oj2/contract"
+import type { TutorialProgress } from "utils/types"
+
+const props = defineProps<{
+  titles: { id: number; title: string }[]
+  progress: Record<number, TutorialProgress>
+  traced: boolean
+}>()
+
+const stats = computed(() => {
+  const rows = props.titles.map((t) => props.progress[t.id])
+  const read = rows.filter(
+    (p) => p && p.totalSeconds >= TUTORIAL_READ_SECONDS,
+  ).length
+  const solved = rows.reduce((n, p) => n + (p?.exerciseSolved ?? 0), 0)
+  const total = rows.reduce((n, p) => n + (p?.exerciseTotal ?? 0), 0)
+  return { read, solved, total }
+})
+
+const percent = computed(() =>
+  props.titles.length
+    ? Math.round((stats.value.read / props.titles.length) * 100)
+    : 0,
+)
+</script>
+
+<template>
+  <div v-if="traced && titles.length" class="summary">
+    <n-progress
+      type="line"
+      :percentage="percent"
+      :height="8"
+      :show-indicator="false"
+      status="success"
+    />
+    <n-text depth="3" class="numbers">
+      已读 {{ stats.read }}/{{ titles.length }} 课
+      <template v-if="stats.total">
+        · 练一练 {{ stats.solved }}/{{ stats.total }}
+      </template>
+    </n-text>
+  </div>
+</template>
+
+<style scoped>
+.summary {
+  padding: 4px 10px 12px;
+}
+.numbers {
+  display: block;
+  margin-top: 6px;
+  font-size: 12px;
+}
+</style>