优化: 无障碍第二批(icon-only aria-label + 容器键盘化 20 处)

- TopBar 10 处 icon-only 按钮补 aria-label(复用同 :title 表达式,含三态 title)
- ConversationSidebar: 4 icon-only aria-label + 2 分组标题键盘化(role/tabindex/aria-expanded/keydown)
- MessageList 2 icon-only; KnowledgeDetail 3 章节折叠键盘化; dashboard 两列表项键盘化
- FileExplorer 面包屑/模块下拉项键盘化; ConflictResolver 冲突文件折叠键盘化
- 约定: 遮罩/点外关闭类(@click.self)非键盘缺口,跳过;零新增 i18n 键
- 验收: bunx vue-tsc --noEmit EXIT 0;看板 P3-2 更新(两批共 23 处)

src/components/ai/MessageList.vue

@@ -1877,7 +1877,7 @@ defineExpose({
   <!-- 回到底部（上滑看历史 / 有新内容时显示） -->
   <!-- R11(2026-10-05):生成中且非跟随态 → 按钮加脉冲点,提示「下面有新内容」而非仅导航 -->
   <Transition name="back-to-bottom">
-    <button v-if="showBackToBottom" class="ai-back-to-bottom" :class="{ 'has-new': store.state.streaming && !isFollowingBottom }" @click="() => scrollToBottom()" :title="$t('aiChat.backToBottom')">
+    <button v-if="showBackToBottom" class="ai-back-to-bottom" :class="{ 'has-new': store.state.streaming && !isFollowingBottom }" @click="() => scrollToBottom()" :title="$t('aiChat.backToBottom')" :aria-label="$t('aiChat.backToBottom')">
       <svg aria-hidden="true" width="14" height="14" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><line x1="12" y1="5" x2="12" y2="19"/><polyline points="19 12 12 19 5 12"/></svg>
       <span v-if="store.state.streaming && !isFollowingBottom" class="ai-btb-new-dot"></span>
     </button>
@@ -1897,7 +1897,7 @@ defineExpose({
             <span v-if="agentPeekSubject.task" class="ai-agent-peek-task">{{ agentPeekSubject.task }}</span>
           </div>
         </div>
-        <button class="ai-agent-peek-close" :title="$t('common.close')" @click="closeAgentTrace">
+        <button class="ai-agent-peek-close" :title="$t('common.close')" :aria-label="$t('common.close')" @click="closeAgentTrace">
           <svg aria-hidden="true" width="12" height="12" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><line x1="18" y1="6" x2="6" y2="18"/><line x1="6" y1="6" x2="18" y2="18"/></svg>
         </button>
       </div>