fix: 移除所有硬编码的localhost:8080，使用相对路径

- 创建utils/config.ts集中管理API_BASE和文件URL
- 删除.env文件（VITE_SERVER_API_URL=http://localhost:8080）
- env.ts默认API前缀改为/api/v1匹配后端路由
- 替换16处硬编码的localhost:8080引用

frontend/src/layouts/admin.vue

@@ -7,6 +7,7 @@ import LanguageChange from '@/components/language-change.vue'
 import ThemePopover from '@/components/custom-theme/theme-popover.vue'
 import ToggleTheme from '@/components/toggle-theme.vue'
 import api from '@/services/api'
+import { getFileUrl } from '@/utils/config'
 
 const router = useRouter()
 
@@ -72,11 +73,7 @@ const breadcrumbs = computed(() => {
 })
 
 function getAssetUrl(path: string) {
-  if (!path)
-    return ''
-  if (path.startsWith('http'))
-    return path
-  return `${import.meta.env.VITE_API_BASE || 'http://localhost:8080'}${path}`
+  return getFileUrl(path)
 }
 
 function updateFavicon(favicon: string) {