Enhance mini program environment configuration and chapter loading logic

- Introduced a debugging environment configuration in app.js, allowing for dynamic API endpoint selection based on the environment.
- Implemented lazy loading for chapter parts in chapters.js, improving performance by only fetching necessary data.
- Updated UI elements in chapters.wxml to reflect changes in chapter loading and added loading indicators.
- Enhanced error handling and data management for chapter retrieval, ensuring a smoother user experience.
- Added functionality for switching API environments in settings.js, visible only in development mode.

miniprogram/pages/chapters/chapters.wxml

@@ -34,8 +34,8 @@
 
   <!-- 目录内容 -->
   <view class="chapters-content">
-    <!-- 序言 -->
-    <view class="chapter-item" bindtap="goToRead" data-id="preface">
+    <!-- 序言（优先传 mid，阅读页用 by-mid 请求） -->
+    <view class="chapter-item" bindtap="goToRead" data-id="preface" data-mid="{{fixedSectionsMap.preface}}">
       <view class="item-left">
         <view class="item-icon icon-brand">📖</view>
         <text class="item-title">序言｜为什么我每天早上6点在Soul开播?</text>
@@ -59,14 +59,15 @@
             </view>
           </view>
           <view class="part-right">
-            <text class="part-count">{{item.chapters.length}}章</text>
+            <text class="part-count">{{item.chapters.length || item.chapterCount}}章</text>
             <text class="part-arrow {{expandedPart === item.id ? 'arrow-down' : ''}}">→</text>
           </view>
         </view>
 
-        <!-- 章节列表 - 展开时显示 -->
+        <!-- 章节列表 - 展开时显示，懒加载 -->
         <block wx:if="{{expandedPart === item.id}}">
           <view class="chapters-list">
+            <view wx:if="{{item.chapters.length === 0}}" class="chapters-loading">加载中...</view>
             <block wx:for="{{item.chapters}}" wx:key="id" wx:for-item="chapter">
               <view class="chapter-header">{{chapter.title}}</view>
               <view class="section-list">
@@ -92,8 +93,8 @@
       </view>
     </view>
 
-    <!-- 尾声 -->
-    <view class="chapter-item" bindtap="goToRead" data-id="epilogue">
+    <!-- 尾声（优先传 mid） -->
+    <view class="chapter-item" bindtap="goToRead" data-id="epilogue" data-mid="{{fixedSectionsMap.epilogue}}">
       <view class="item-left">
         <view class="item-icon icon-brand">📖</view>
         <text class="item-title">尾声｜这本书的真实目的</text>
@@ -114,6 +115,7 @@
           wx:key="id"
           bindtap="goToRead"
           data-id="{{item.id}}"
+          data-mid="{{item.mid}}"
         >
           <text class="appendix-text">{{item.title}}</text>
           <text class="appendix-arrow">→</text>