feat: 视频缩略图生成、最近文件面板与前端视图重构

- 新增 VideoThumbnailService，基于 ffmpeg 截取视频缩略图，ffprobe 提取时长
  - 新增 ManagedThumbnailMap 模型及多数据库迁移，存储缩略图元数据
  - 新增 /api/thumbnails/{id} 缩略图流端点
  - 新增最近添加/最近播放 API 与前端面板，支持列表/网格双视图切换
  - FileRecordDto 扩展 thumbnailUrl、videoDuration、lastPlayedAt 字段
  - 前端新增文件库 Tab 导航、卡片网格视图、视频海报与时长信息栏
  - 添加文件库目录不再同步全量扫描，改为后台异步自动扫描

FileShare-API/Configuration/ServicesConfiguration.cs

@@ -40,6 +40,14 @@ namespace FileShare_API.Configuration
 
             // ---- 业务服务 ----
             services.AddScoped<WeatherForecastService>();
+            var thumbnailOptions = configuration
+                .GetSection(nameof(ThumbnailStorageOptions))
+                .Get<ThumbnailStorageOptions>()
+                ?? new ThumbnailStorageOptions();
+            services.AddSingleton(thumbnailOptions);
+            services.AddSingleton<IVideoThumbnailService>(sp =>
+                new VideoThumbnailService(sp.GetRequiredService<ThumbnailStorageOptions>()));
+            services.AddScoped<IThumbnailStreamService, ThumbnailStreamService>();
             services.AddScoped<IFileLibraryService, FileLibraryService>();
             services.AddScoped<IFileLibraryEndpointService, FileLibraryEndpointService>();
             services.AddScoped<IFileStreamService, FileStreamService>();