fix(mobile): replace emoji glyphs with uni-icons and flex centering app-wide

- ChatComposer panel contract: callers now pass uni-icons type names
  (image/sound/list/help/checkmark) instead of emoji
- voice playback buttons: sound icon + pulse animation, no more 🔊/◼ chars
- practice recent list: rare symbols ⌕⌁◉ (tofu risk) -> wallet/gear/chatbubble
- sop resource icon ▶/文 -> 视/文 character badges
- back arrows: 22 char-based ← -> uni-icons back across 21 pages
- line-height centering -> flex in 8 spots (back buttons, media-remove,
  turn-voice capsule, option-marker, text-button, play-question)

mobile-uni/src/pages/supervisor/exams/edit.vue

@@ -1,7 +1,7 @@
 <template>
   <view class="page exam-edit-page">
     <view class="subpage-header">
-      <button class="subpage-back-button" aria-label="返回考试列表" :disabled="saving" @click="goBack">←</button>
+      <button class="subpage-back-button" aria-label="返回考试列表" :disabled="saving" @click="goBack"><uni-icons type="back" size="20" color="#25282d" /></button>
       <view class="subpage-heading"><text class="page-title">{{ examId ? '编辑考试' : '新建考试' }}</text><text class="page-subtitle">发布后冻结题目版本，已派发试卷不随草稿变化。</text></view>
     </view>
     <text v-if="message" class="status-message">{{ message }}</text>

mobile-uni/src/pages/supervisor/exams/index.vue

@@ -1,7 +1,7 @@
 <template>
   <view class="page supervisor-exams-page">
     <view class="subpage-header">
-      <button class="subpage-back-button" aria-label="返回主管首页" @click="goBack">←</button>
+      <button class="subpage-back-button" aria-label="返回主管首页" @click="goBack"><uni-icons type="back" size="20" color="#25282d" /></button>
       <view class="subpage-heading"><text class="page-title">岗前测试</text><text class="page-subtitle">按岗位组卷、选择真实员工并查看完成情况。</text></view>
     </view>
     <button class="primary-button create-button" @click="createExam">新建考试</button>

mobile-uni/src/pages/supervisor/exams/results.vue

@@ -1,6 +1,6 @@
 <template>
   <view class="page exam-results-page">
-    <view class="subpage-header"><button class="subpage-back-button" aria-label="返回" @click="goBack">←</button><view class="subpage-heading"><text class="page-title">考试结果</text><text class="page-subtitle">按人员查看成绩、未完成与通过状态。</text></view></view>
+    <view class="subpage-header"><button class="subpage-back-button" aria-label="返回" @click="goBack"><uni-icons type="back" size="20" color="#25282d" /></button><view class="subpage-heading"><text class="page-title">考试结果</text><text class="page-subtitle">按人员查看成绩、未完成与通过状态。</text></view></view>
     <text v-if="message" class="status-message">{{ message }}</text>
     <view class="metric-grid">
       <view class="metric-card"><text>{{ results.length }}</text><label>已派发</label></view>