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-EFCore/Models/ManagedFileRecord.cs

@@ -67,6 +67,18 @@ namespace FileShare_EFCore.Models
         [Column("last-seen-at")]
         public DateTime LastSeenAt { get; set; } = DateTime.UtcNow;
 
+        /// <summary>视频缩略图路径（相对于 wwwroot）。</summary>
+        [Column("thumbnail-id")]
+        public int? ThumbnailId { get; set; }
+
+        /// <summary>视频时长（秒）。</summary>
+        [Column("video-duration")]
+        public double? VideoDuration { get; set; }
+
+        /// <summary>最近一次播放时间 UTC。</summary>
+        [Column("last-played-at")]
+        public DateTime? LastPlayedAt { get; set; }
+
         /// <summary>创建时间。</summary>
         [Column("created-at")]
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
@@ -77,5 +89,7 @@ namespace FileShare_EFCore.Models
 
         /// <summary>所属根目录。</summary>
         public ManagedLibraryRoot? LibraryRoot { get; set; }
+
+        public ManagedThumbnailMap? Thumbnail { get; set; }
     }
 }

FileShare-EFCore/Models/ManagedLibraryRoot.cs

@@ -62,5 +62,7 @@ namespace FileShare_EFCore.Models
 
         /// <summary>文件记录。</summary>
         public List<ManagedFileRecord> Files { get; set; } = new();
+
+        public List<ManagedThumbnailMap> Thumbnails { get; set; } = new();
     }
 }

FileShare-EFCore/Models/ManagedThumbnailMap.cs

@@ -0,0 +1,37 @@
+using Microsoft.EntityFrameworkCore;
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace FileShare_EFCore.Models
+{
+    [Comment("文件缩略图映射记录")]
+    [Table("managed-thumbnail-map")]
+    public class ManagedThumbnailMap
+    {
+        [Key]
+        [Column("id")]
+        [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
+        public int Id { get; set; }
+
+        [Column("library-root-id")]
+        public int LibraryRootId { get; set; }
+
+        [Column("relative-path")]
+        [MaxLength(1024)]
+        public string RelativePath { get; set; } = string.Empty;
+
+        [Column("content-type")]
+        [MaxLength(100)]
+        public string ContentType { get; set; } = "image/jpeg";
+
+        [Column("created-at")]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Column("updated-at")]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        public ManagedLibraryRoot? LibraryRoot { get; set; }
+
+        public List<ManagedFileRecord> Files { get; set; } = new();
+    }
+}