feat(style): 添加首页背景图片配置选项并优化设置文件格式

- 在 scss 文件中添加了 $home_bg_image 变量用于背景图片配置
- 将 settings.yml 中的列表默认值格式从数组改为多行字符串格式
- 更新 extra_glass_morphism_selectors 和 extra_card_item_selectors 的默认值格式
- 保持所有选择器功能不变，仅调整配置文件的可读性
- 为首页背景功能提供了默认空值配置选项

scss/self.scss

@@ -1,5 +1,7 @@
 @use "lib/viewport";
 
+$home_bg_image: "" !default;
+
 #main-outlet {
   @if $home_bg_image != "" {
     background-image: url($home_bg_image);

settings.yml

@@ -17,28 +17,28 @@ home_bg_image:
 
 extra_glass_morphism_selectors:
   type: list
-  default:
-    - .regular.ember-view
-    - .user-main
-    - .reviewable
-    - .admin-content
-    - .contents.clearfix.body-page
-    - .search-container
-    - .show-badge
-    - .users-directory
-    - "#main-outlet>.edit-category"
-    - .container.groups-index
-    - .container.group
+  default: |
+    .regular.ember-view
+    .user-main
+    .reviewable
+    .admin-content
+    .contents.clearfix.body-page
+    .search-container
+    .show-badge
+    .users-directory
+    #main-outlet>.edit-category
+    .container.groups-index
+    .container.group
   description: "额外应用毛玻璃效果的选择器列表"
 
 extra_card_item_selectors:
   type: list
-  default:
-    - .topic-post.clearfix.regular
-    - .post-list-item.user-stream-item
-    - .search-header
-    - .admin-plugin-config-page__content
-    - ".user-main .about.collapsed-info .details"
-    - .ember-view.group-box
+  default: |
+    .topic-post.clearfix.regular
+    .post-list-item.user-stream-item
+    .search-header
+    .admin-plugin-config-page__content
+    .user-main .about.collapsed-info .details
+    .ember-view.group-box
   description: "额外应用卡片项目样式的选择器列表"