fix(widgets): keep widget-content an unstyled structural layer

Reference contract: widget cards carry their own 15px padding and
.widget-content is bare. The added 20px padding inset every widget
card from the content max width on the fixed homepage (the only
bundle loading sidebar.css).

- drop the root padding rule and the unreferenced --flush modifier
- drop the now-redundant user-widget padding:0 workaround
- drop the dead --card-padding-lg token
- lock the contract with negative assertions in the display test

assets/css/frontend/sidebar.css

@@ -36,13 +36,8 @@
   font-size: 14px;
 }
 
-.widget-content {
-  padding: var(--card-padding-lg, 20px);
-}
-
-.widget-content--flush {
-  padding: 0;
-}
+/* .widget-content 是无样式结构层（对齐参考主题）：小工具卡片自带内边距，
+ * 在这里加 padding 会让所有小工具与内容列宽产生左右空隙。 */
 
 /* Widget 链接 */
 .widget a {