refactor(app): 重构应用主题和页面结构

- 移除动态主题切换功能，改为全局统一深色主题
- 实现自定义TabBar组件替换原生tabBar，解决iOS闪白问题
- 创建main容器页面统一管理三个tab页面的生命周期和状态
- 将所有页面的onShow/onHide等生命周期方法迁移到子组件内部
- 更新页面路径从index改为main，并调整路由跳转逻辑
- 移除theme-mixin和相关主题工具函数
- 统一页面背景色设置，优化用户体验一致性

pages/circle-publish/circle-publish.vue

@@ -1,5 +1,5 @@
 <template>
-  <view :class="themeClass" class="page-container publish-page">
+  <view class="page-container publish-page">
     <!-- 顶部导航 -->
     <view class="publish-nav" :style="{ paddingTop: navPaddingTop, paddingRight: navPaddingRight }">
       <view class="publish-nav-inner">
@@ -81,10 +81,8 @@
 <script>
 import { PublishFeed, UploadImage } from '../../common/api'
 import client from '../../common/api'
-import themeMixin from '../../common/theme-mixin'
 
 export default {
-  mixins: [themeMixin],
   data() {
     const menuBtn = uni.getMenuButtonBoundingClientRect()
     const sysInfo = uni.getSystemInfoSync()