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-PC/Views/MainWindow.axaml.cs

@@ -554,7 +554,8 @@ namespace FileShare_PC.Views
         {
             try
             {
-                if (await TryHandleLocalMediaStreamAsync(context))
+                if (await TryHandleLocalMediaStreamAsync(context)
+                    || await TryHandleLocalThumbnailAsync(context))
                 {
                     return;
                 }
@@ -691,6 +692,69 @@ namespace FileShare_PC.Views
             return true;
         }
 
+        private async Task<bool> TryHandleLocalThumbnailAsync(HttpListenerContext context)
+        {
+            var request = context.Request;
+            var response = context.Response;
+            var segments = (request.Url?.AbsolutePath ?? string.Empty)
+                .Split('/', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);
+
+            if (segments.Length != 3
+                || !string.Equals(segments[0], "api", StringComparison.OrdinalIgnoreCase)
+                || !string.Equals(segments[1], "thumbnails", StringComparison.OrdinalIgnoreCase))
+            {
+                return false;
+            }
+
+            AddLocalMediaHeaders(response);
+
+            if (!string.Equals(request.HttpMethod, "GET", StringComparison.OrdinalIgnoreCase)
+                && !string.Equals(request.HttpMethod, "HEAD", StringComparison.OrdinalIgnoreCase))
+            {
+                response.StatusCode = (int)HttpStatusCode.MethodNotAllowed;
+                response.Close();
+                return true;
+            }
+
+            if (!int.TryParse(segments[2], out var id) || id <= 0)
+            {
+                response.StatusCode = (int)HttpStatusCode.NotFound;
+                response.Close();
+                return true;
+            }
+
+            using var scope = _services.CreateScope();
+            var thumbnailService = scope.ServiceProvider.GetService<IThumbnailStreamService>();
+            var thumbnail = thumbnailService is null ? null : await thumbnailService.GetThumbnailAsync(id);
+            if (thumbnail is null || !File.Exists(thumbnail.FilePath))
+            {
+                response.StatusCode = (int)HttpStatusCode.NotFound;
+                response.Close();
+                return true;
+            }
+
+            var fileInfo = new FileInfo(thumbnail.FilePath);
+            response.StatusCode = (int)HttpStatusCode.OK;
+            response.ContentType = thumbnail.ContentType;
+            response.ContentLength64 = fileInfo.Length;
+            response.Headers["Cache-Control"] = "public, max-age=3600";
+            response.Headers["Content-Disposition"] =
+                $"inline; filename=\"{Uri.EscapeDataString(thumbnail.FileName)}\"";
+            response.Headers["Last-Modified"] = thumbnail.LastModified.ToUniversalTime().ToString("R");
+
+            if (string.Equals(request.HttpMethod, "HEAD", StringComparison.OrdinalIgnoreCase)
+                || fileInfo.Length == 0)
+            {
+                response.Close();
+                return true;
+            }
+
+            await using var input = File.Open(thumbnail.FilePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
+            await input.CopyToAsync(response.OutputStream);
+            response.OutputStream.Close();
+            return true;
+        }
+
         /// <summary>
         /// 为媒体流响应添加 CORS 和 Range 请求头，允许浏览器跨域访问和分段请求。
         /// </summary>