style(h5): 大类卡片背景图改为本地资源

- 从 Pexels 下载建筑/景观/设备/装修四张主题图存到 public/img/majors/
- 不再依赖外部 CDN，离线/弱网也能正常显示

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend-h5/src/views/Home.vue

@@ -20,11 +20,12 @@ const fallbackMajors = [
   { value: 'decoration', label: '装修' },
 ]
 
+const BASE = import.meta.env.BASE_URL
 const majorBg = {
-  architecture: 'https://picsum.photos/seed/mat3-architecture/600/300',
-  landscape: 'https://picsum.photos/seed/mat3-landscape/600/300',
-  equipment: 'https://picsum.photos/seed/mat3-equipment/600/300',
-  decoration: 'https://picsum.photos/seed/mat3-decoration/600/300',
+  architecture: `${BASE}img/majors/architecture.jpg`,
+  landscape: `${BASE}img/majors/landscape.jpg`,
+  equipment: `${BASE}img/majors/equipment.jpg`,
+  decoration: `${BASE}img/majors/decoration.jpg`,
 }
 
 const tree = ref([])