fix(scss): 添加缺失的语义化颜色变量别名

添加 $color-bg-secondary, $color-bg-tertiary, $color-border,
$color-text-secondary, $color-text-tertiary 变量别名，
修复 dashboard.module.scss 和 TrendChart.module.scss 构建错误。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/admin-web/src/styles/variables.scss

@@ -117,3 +117,12 @@ $border-color-light: #F0F0F0;
 
 // 文本颜色
 $text-placeholder: #9E9E9E;
+
+// ============================================
+// 语义化颜色别名 (用于骨架屏/加载状态等)
+// ============================================
+$color-bg-secondary: $background-color;
+$color-bg-tertiary: lighten($background-color, 3%);
+$color-border: $border-color;
+$color-text-secondary: $text-secondary;
+$color-text-tertiary: $text-disabled;