style: 统一主容器背景为深色主题

将所有主要容器背景色统一改为 --color-bg-primary，
实现全站深色主题的视觉一致性：
- AppHeader: 头部导航栏
- MainLayout: 主布局侧边栏和工作区头部
- CadSidebar: 左侧边栏容器
- CreoModelViewer: 模型查看器主容器
- ModelManagement: 模型管理主容器

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

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

src/components/layout/AppHeader.vue

@@ -110,7 +110,7 @@ const handleNavClick = (page) => {
   align-items: center;
   justify-content: space-between;
   height: 60px;
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   border-bottom: 1px solid var(--color-border-primary);
   padding: 0 20px;
   position: relative;

src/components/layout/CadSidebar.vue

@@ -165,7 +165,7 @@ const handleOpenModel = async () => {
   height: 100%;
   display: flex;
   flex-direction: column;
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   color: var(--color-text-primary);
   overflow-y: auto;
   padding: 20px;

src/components/layout/MainLayout.vue

@@ -81,7 +81,7 @@ import AppHeader from './AppHeader.vue'
 
 .app-sidebar {
   width: 350px;
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   border-right: 1px solid var(--color-border-primary);
   overflow-y: auto;
   flex-shrink: 0;
@@ -100,7 +100,7 @@ import AppHeader from './AppHeader.vue'
   align-items: center;
   justify-content: space-between;
   padding: 16px 24px;
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   border-bottom: 1px solid var(--color-border-primary);
   flex-shrink: 0;
 }

src/components/layout/ModelManagement.vue

@@ -572,7 +572,7 @@ const openSelectedFile = async () => {
 }
 
 .file-dialog {
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   border: 1px solid var(--color-border-primary);
   border-radius: var(--size-border-radius-card);
   width: 90%;

src/components/model/CreoModelViewer.vue

@@ -189,7 +189,7 @@ const basicCheck = () => {
 <style scoped>
 .project-info-dashboard {
   padding: 20px;
-  background: var(--color-bg-secondary);
+  background: var(--color-bg-primary);
   border-radius: 8px;
   min-height: 500px;
 }