修复 H5 嵌入小程序导航栏冲突

- 识别微信小程序 web-view 宿主环境
- 宿主导航存在时隐藏 H5 自绘返回与页面标题
- 收回详情、搜索、讲解列表页面顶部预留空间

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

src/components/explain/ExplainHallSelect.vue

@@ -1,6 +1,6 @@
 <template>
-  <view class="explain-hall-select">
-    <view class="explain-page-header">
+  <view class="explain-hall-select" :class="{ 'host-navigation': shouldUseHostNavigation }">
+    <view v-if="!shouldUseHostNavigation" class="explain-page-header">
       <view class="header-back" @tap="handleBack">
         <text class="header-back-icon">‹</text>
         <text class="header-back-text">返回</text>
@@ -153,6 +153,7 @@ import { computed, ref } from 'vue'
 import {
   EXPLAIN_UNIT_PREVIEW_FALLBACK
 } from '@/utils/explainUnitPreviews'
+import { isEmbeddedInWechatMiniProgram } from '@/utils/hostEnvironment'
 
 export interface ExplainHallSelectItem {
   id: string
@@ -239,6 +240,7 @@ const hallPreviewMap: Record<string, string> = {
 }
 
 const filteredHalls = computed(() => props.halls)
+const shouldUseHostNavigation = computed(() => isEmbeddedInWechatMiniProgram())
 const headerTitle = computed(() => {
   if (props.stage === 'unit') return props.selectedHallName || '选择业务单元'
   if (props.stage === 'stop') return props.selectedBusinessUnitName || '选择讲解点'
@@ -356,6 +358,14 @@ const handleBack = () => {
   padding-bottom: 0;
 }
 
+.explain-hall-select.host-navigation .explain-scroll {
+  padding-top: 12px;
+}
+
+.explain-hall-select.host-navigation .explain-scroll.hall-stage {
+  padding-top: 12px;
+}
+
 .state-title,
 .state-desc,
 .hall-name,