refactor(styles): 提取布局 mixin，派生主题色，消除硬编码与魔法数字

- 新增 _mixins.scss，统一 app-main / sidebar-scroll 高度计算
- _theme.scss 引入 SCSS 颜色令牌，menu-hover 改用 color-mix() 自动派生
- _variables.scss 新增 $sidebar-toggle-height 替代魔法数字 50px
- LeftLayout / TopLayout / MixLayout 中重复 calc 替换为 mixin 调用
- 修复 apifox / internal-doc demo 页面 tags-view 高度错误（34px→40px）
- 添加主题变量契约注释，方便后续新增主题

src/layouts/LeftLayout.vue

@@ -45,6 +45,8 @@ const appStore = useAppStore();
 </script>
 
 <style lang="scss" scoped>
+@use "@/styles/base/mixins" as *;
+
 .layout {
   &__sidebar {
     position: fixed;
@@ -69,7 +71,7 @@ const appStore = useAppStore();
 
       &.has-logo {
         .el-scrollbar {
-          height: calc(100vh - $navbar-height);
+          @include sidebar-scroll-height-with-logo;
         }
       }
 
@@ -131,7 +133,7 @@ const appStore = useAppStore();
 
 .hasTagsView {
   :deep(.app-main) {
-    height: calc(100vh - $navbar-height - $tags-view-height) !important;
+    @include app-main-height-with-tags;
   }
 }
 </style>