feat(theme): 添加首页背景图片功能

- 在设置中新增 home_bg_image 配置选项
- 实现主题 SCSS 中的背景图片样式逻辑
- 添加对首页背景图片的条件渲染支持
- 集成上传功能以支持自定义背景图片
- 设置默认值为空字符串以避免初始加载问题
- 添加中文描述文档以便用户理解功能用途

scss/topic-cards.scss

@@ -765,6 +765,14 @@ body.user-messages-page {
   position: relative;
 }
 
+#main-outlet {
+  @if $home_bg_image != "" {
+    background-image: url($home_bg_image);
+    background-size: cover;
+    background-position: center;
+  }
+}
+
 .container.list-container{
   position: relative;
 }

settings.yml

@@ -9,3 +9,9 @@ search_experience:
     - search_field
     - search_icon
   description: "Overrides the core `search experience` site setting"
+
+home_bg_image:
+  type: upload
+  default: ""
+  description: "首页背景图"
+