feat(mobile): learning page APP-ification (fold long content, batch 2 of app-like)

学习中心页(全站最长,实测 5452px)按「折叠不删减」治理:
- 岗位 SOP 长答案默认收起为 140 字预览+「展开全文/收起全文」
  (短答案不出现开关;切换主题自动复位)
- 学习资料段落(snippet)默认只显示标题行,点标题展开/收起正文;
  超长文档标题两行截断
- 错题本默认 3 条,「展开全部 N 条」可切换(随归因筛选联动)

实测页面高度 5452 → 3834(-30%),其余为八个功能面板固有高度,
继续压缩需删减面板,违背不影响功能原则,不做。
本地+生产复核:高度、段落默认收起、点击展开均通过。
发布备份 h5-before-$TS。

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

mobile-uni/src/pages/user/learning/index.vue

@@ -98,7 +98,7 @@
         </button>
       </view>
       <view v-if="visibleMistakes.length" class="mistake-list">
-        <view v-for="item in visibleMistakes" :key="item.id" class="mistake-row">
+        <view v-for="item in displayedMistakes" :key="item.id" class="mistake-row">
           <view class="mistake-copy">
             <view class="mistake-meta">
               <text class="mistake-cause">{{ item.cause }}</text>
@@ -117,6 +117,9 @@
           </button>
           <text v-else-if="item.retryAssignmentId" class="mistake-completed">已完成</text>
         </view>
+        <button v-if="visibleMistakes.length > 3" class="fold-toggle" @click="showAllMistakes = !showAllMistakes">
+          {{ showAllMistakes ? '收起 ▲' : `展开全部 ${visibleMistakes.length} 条 ▼` }}
+        </button>
       </view>
       <view v-else-if="!mistakeLoading" class="empty">
         {{ mistakes.length ? '当前归因筛选暂无错题' : '完成训练后，低分和红线回合会自动出现在这里' }}
@@ -224,16 +227,26 @@
           {{ topic }}
         </button>
       </view>
-      <text v-if="knowledgeResult?.answer" class="answer-text">{{ knowledgeResult.answer }}</text>
+      <text v-if="knowledgeResult?.answer" class="answer-text">{{ displayedAnswer }}</text>
+      <button v-if="answerIsLong" class="fold-toggle" @click="showFullAnswer = !showFullAnswer">
+        {{ showFullAnswer ? '收起全文 ▲' : '展开全文 ▼' }}
+      </button>
       <view v-if="knowledgeResult?.keyPoints?.length" class="keypoints">
         <text v-for="(point, index) in knowledgeResult.keyPoints" :key="point" class="keypoint">
           {{ index + 1 }}. {{ cleanOrderedText(point) }}
         </text>
       </view>
       <view v-if="knowledgeResult?.snippets?.length" class="snippet-list">
-        <view v-for="(snippet, index) in knowledgeResult.snippets?.slice(0, 3)" :key="`${snippet.docTitle}-${index}`" class="snippet-row">
-          <text class="snippet-title">{{ snippet.docTitle || '知识片段' }}</text>
-          <text class="snippet-content">{{ snippetExcerpt(snippet.content) }}</text>
+        <view
+          v-for="(snippet, index) in knowledgeResult.snippets?.slice(0, 3)"
+          :key="`${snippet.docTitle}-${index}`"
+          class="snippet-row"
+        >
+          <button class="snippet-head" @click="toggleSnippet(index)">
+            <text class="snippet-title">{{ snippet.docTitle || '知识片段' }}</text>
+            <text class="snippet-arrow">{{ expandedSnippets[index] ? '▲' : '▼' }}</text>
+          </button>
+          <text v-if="expandedSnippets[index]" class="snippet-content">{{ snippetExcerpt(snippet.content) }}</text>
         </view>
       </view>
       <view v-else-if="!knowledgeLoading" class="empty">知识库暂未命中可引用内容</view>
@@ -348,6 +361,22 @@ const visibleMistakes = computed(() => activeMistakeCause.value === '全部'
   ? mistakes.value
   : mistakes.value.filter((item) => item.cause === activeMistakeCause.value));
 
+/* APP-like folding: long content collapsed by default, nothing removed. */
+const ANSWER_PREVIEW_CHARS = 140;
+const showAllMistakes = ref(false);
+const showFullAnswer = ref(false);
+const expandedSnippets = ref<Record<number, boolean>>({});
+const displayedMistakes = computed(() => (showAllMistakes.value ? visibleMistakes.value : visibleMistakes.value.slice(0, 3)));
+const answerIsLong = computed(() => (knowledgeResult.value?.answer || '').length > ANSWER_PREVIEW_CHARS);
+const displayedAnswer = computed(() => {
+  const answer = knowledgeResult.value?.answer || '';
+  if (showFullAnswer.value || answer.length <= ANSWER_PREVIEW_CHARS) return answer;
+  return `${answer.slice(0, ANSWER_PREVIEW_CHARS)}…`;
+});
+const toggleSnippet = (index: number) => {
+  expandedSnippets.value = { ...expandedSnippets.value, [index]: !expandedSnippets.value[index] };
+};
+
 const qualificationStatusLabel = computed(() => {
   switch (qualification.value?.status) {
     case 'PASSED': return '已通过';
@@ -554,6 +583,8 @@ const loadKnowledge = async (topic: string, request: RequestScopeTicket = reques
     const nextKnowledge = await searchKnowledge(topic);
     if (!requests.isCurrent(request)) return;
     knowledgeResult.value = nextKnowledge;
+    showFullAnswer.value = false;
+    expandedSnippets.value = {};
   } catch (error) {
     if (!requests.isCurrent(request)) return;
     knowledgeResult.value = null;
@@ -1157,4 +1188,53 @@ onUnload(invalidateRequests);
 .action-button {
   width: 100%;
 }
+
+.fold-toggle {
+  display: block;
+  width: 100%;
+  margin: 10px 0 0;
+  padding: 8px 0;
+  border: none;
+  border-top: 1px dashed #eceff2;
+  background: transparent;
+  color: #6b7480;
+  font-size: 13px;
+  text-align: center;
+  line-height: 1.5;
+}
+
+.fold-toggle::after {
+  border: none;
+}
+
+.snippet-head {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  width: 100%;
+  margin: 0;
+  padding: 0;
+  border: none;
+  background: transparent;
+  text-align: left;
+  line-height: 1.4;
+}
+
+.snippet-head::after {
+  border: none;
+}
+
+.snippet-arrow {
+  flex-shrink: 0;
+  color: #98a1ad;
+  font-size: 12px;
+}
+
+.snippet-head .snippet-title {
+  display: -webkit-box;
+  -webkit-box-orient: vertical;
+  -webkit-line-clamp: 2;
+  overflow: hidden;
+}
 </style>