refactor: migrate from @import to @use for SCSS files and clean up transition properties in layout

- Update SCSS files to use @use instead of @import for better modularity
- Refactor transition properties in layout to improve readability and maintainability

src/assets/styles/index.scss

@@ -1,7 +1,7 @@
 @use './variables.scss' as *;
 
 // 导入主题系统
-@import './theme/index.scss';
+@use './theme/index.scss';
 
 * {
   margin: 0;

src/assets/styles/theme/index.scss

@@ -3,7 +3,7 @@
  */
 
 // CSS 变量定义
-@import './css-variables.scss';
+@use './css-variables.scss';
 
 // Element Plus 覆盖
-@import './element-override.scss';
+@use './element-override.scss';

src/layout/index.vue

@@ -288,8 +288,7 @@ onMounted(() => {
 
 .sidebar-container {
   background-color: var(--bg-sidebar);
-  transition:
-    width var(--transition-base) var(--transition-timing),
+  transition: width var(--transition-base) var(--transition-timing),
     background-color var(--transition-base) var(--transition-timing);
   overflow: auto;
 
@@ -337,8 +336,7 @@ onMounted(() => {
   height: var(--header-height);
   background-color: var(--bg-header);
   box-shadow: var(--shadow-header);
-  transition:
-    background-color var(--transition-base) var(--transition-timing),
+  transition: background-color var(--transition-base) var(--transition-timing),
     box-shadow var(--transition-base) var(--transition-timing);
 
   .header-left {