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/user/questions/detail.vue

@@ -1,7 +1,7 @@
 <template>
   <view class="page employee-page-shell detail-page">
     <view class="subpage-header">
-      <button class="subpage-back-button" aria-label="返回问题榜" @click="goBack">←</button>
+      <button class="subpage-back-button" aria-label="返回问题榜" @click="goBack"><uni-icons type="back" size="20" color="#25282d" /></button>
       <view class="subpage-heading">
         <text class="page-title">问题详情</text>
         <text class="page-subtitle">回答经审核后公开，最佳答案按当前规则获得积分或学习学分。</text>

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

@@ -1,7 +1,7 @@
 <template>
   <view class="page employee-page-shell question-page">
     <view class="subpage-header">
-      <button class="subpage-back-button" aria-label="返回今日" @click="goToday">←</button>
+      <button class="subpage-back-button" aria-label="返回今日" @click="goToday"><uni-icons type="back" size="20" color="#25282d" /></button>
       <view class="subpage-heading">
         <text class="employee-kicker">广谱师傅</text>
         <text class="page-title">开放问题榜</text>

mobile-uni/src/pages/user/questions/publish.vue

@@ -1,7 +1,7 @@
 <template>
   <view class="page employee-page-shell publish-page">
     <view class="subpage-header">
-      <button class="subpage-back-button" aria-label="返回" @click="goBack">←</button>
+      <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">发起求助</text>
         <text class="page-subtitle">员工可发布真实工作问题；有奖征题由运营审核发布。</text>