feat: 前端组件拆分与文件库目录浏览功能

- 将 App.vue 拆分为 AdminPage、ClientPage、QrCodeModal 三个独立组件
- 新增 BrowseDirectory 接口，基于 RelativePath 实现层级目录浏览
- 前端新增面包屑导航、文件夹网格、文件列表等目录浏览 UI
- 新增对应 CSS 样式（breadcrumb、folder-grid、file-list 等）

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

Avalonia-Services/Endpoints/AppEndpoints.cs

@@ -65,6 +65,10 @@ namespace Avalonia_Services.Endpoints
                     .WithOpenApi("FileLibrary", "分页查询已扫描文件。")
                     .WithName("SearchFiles");
 
+                endpoints.MapGet<IFileLibraryEndpointService, BrowseDirectoryRequest>("api/files/browse", (service, request, _) => service.BrowseDirectoryAsync(request))
+                    .WithOpenApi("FileLibrary", "浏览文件库目录结构。")
+                    .WithName("BrowseDirectory");
+
                 endpoints.MapGet<IFileLibraryEndpointService, FileQueryRequest>("api/files/detail", (service, request, _) => service.GetFileAsync(request))
                     .WithOpenApi("FileLibrary", "查询文件详情。")
                     .WithName("GetFileDetail");