refactor: 迁移 sass 模块语法

src/styles/_mixins.scss

@@ -1,4 +1,5 @@
-@import './variables';
+@use 'sass:math';
+@use './variables' as *;
 
 @mixin card($radius: $r-lg) {
   background: $card;
@@ -10,7 +11,7 @@
 @mixin green-gradient-btn($height: 48px) {
   height: $height;
   border: none;
-  border-radius: $height / 2;
+  border-radius: math.div($height, 2);
   font-weight: 700;
   color: #fff;
   background: linear-gradient(135deg, $primary 0%, $primary-dark 100%);