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/settings/settings.wxml

@@ -109,6 +109,15 @@
       <text class="tip-text">提示：绑定微信号才能使用提现功能</text>
     </view>
 
+    <!-- 开发专用：切换 API 环境（仅开发版显示） -->
+    <view class="dev-switch-card" wx:if="{{isDevMode}}" bindtap="switchApiEnv">
+      <view class="dev-switch-inner">
+        <text class="dev-switch-icon">🌐</text>
+        <text class="dev-switch-text">切换环境</text>
+        <text class="dev-switch-desc">当前：{{apiEnvLabel}}（点击循环：自动→本地→测试→正式）</text>
+      </view>
+    </view>
+
     <!-- 开发专用：切换账号（仅开发版显示） -->
     <view class="dev-switch-card" wx:if="{{isDevMode}}" bindtap="openSwitchAccountModal">
       <view class="dev-switch-inner">