refactor: 优化视频预览弹框UI设计

界面优化:
- 将操作按钮改为悬浮在视频右上角的圆形按钮
- 移除底部冗余的操作按钮区域，界面更简洁
- 添加悬停效果和颜色区分：绿色下载、蓝色外链、紫色全屏

用户体验改进:
- 悬浮按钮仅在鼠标悬停时完全显示，不遮挡视频内容
- 下载按钮显示加载动画，提供视觉反馈
- 按钮添加缩放和阴影效果，提升交互体验
- 使用backdrop-blur实现毛玻璃效果

技术实现:
- 使用CSS group-hover实现悬停显示逻辑
- 添加z-index确保按钮层级正确
- 优化transition动画，提供流畅的交互体验
- 移除底部控制栏中重复的全屏按钮

视觉设计:
- 圆形按钮设计更现代化
- 半透明背景不影响视频观看
- 颜色编码帮助用户快速识别功能
- 响应式hover效果提升用户体验

apps/desktop/src/components/VideoPreviewModal.tsx

@@ -218,7 +218,7 @@ export const VideoPreviewModal: React.FC<VideoPreviewModalProps> = ({
     >
       <div className="space-y-4">
         {/* 视频播放区域 */}
-        <div className="relative bg-black rounded-lg overflow-hidden aspect-video">
+        <div className="relative bg-black rounded-lg overflow-hidden aspect-video group">
           {isLoading && (
             <div className="absolute inset-0 flex items-center justify-center bg-gray-900">
               <div className="animate-spin rounded-full h-12 w-12 border-b-2 border-blue-500"></div>
@@ -246,6 +246,40 @@ export const VideoPreviewModal: React.FC<VideoPreviewModalProps> = ({
             controls={false}
           />
 
+          {/* 右上角悬浮按钮组 */}
+          {!isLoading && !error && (
+            <div className="absolute top-4 right-4 flex items-center space-x-2 opacity-80 hover:opacity-100 group-hover:opacity-100 transition-all duration-300 z-10">
+              <button
+                onClick={handleDownload}
+                disabled={isDownloading}
+                className="relative flex items-center justify-center w-10 h-10 bg-black/60 hover:bg-green-600/80 disabled:bg-gray-600/60 text-white rounded-full backdrop-blur-sm transition-all duration-200 hover:scale-110 disabled:cursor-not-allowed shadow-lg"
+                title={isDownloading ? '下载中...' : '下载视频'}
+              >
+                {isDownloading ? (
+                  <div className="animate-spin rounded-full h-5 w-5 border-2 border-white border-t-transparent"></div>
+                ) : (
+                  <Download className="w-5 h-5" />
+                )}
+              </button>
+
+              <button
+                onClick={handleOpenInNewWindow}
+                className="flex items-center justify-center w-10 h-10 bg-black/60 hover:bg-blue-600/80 text-white rounded-full backdrop-blur-sm transition-all duration-200 hover:scale-110 shadow-lg"
+                title="在新窗口打开"
+              >
+                <ExternalLink className="w-5 h-5" />
+              </button>
+
+              <button
+                onClick={handleFullscreen}
+                className="flex items-center justify-center w-10 h-10 bg-black/60 hover:bg-purple-600/80 text-white rounded-full backdrop-blur-sm transition-all duration-200 hover:scale-110 shadow-lg"
+                title="全屏播放"
+              >
+                <Maximize className="w-5 h-5" />
+              </button>
+            </div>
+          )}
+          
           {/* 视频控制覆盖层 */}
           {!isLoading && !error && (
             <div className="absolute inset-0 bg-gradient-to-t from-black/50 via-transparent to-transparent opacity-0 hover:opacity-100 transition-opacity duration-300">
@@ -311,52 +345,12 @@ export const VideoPreviewModal: React.FC<VideoPreviewModalProps> = ({
                         {isMuted ? <VolumeX className="w-5 h-5" /> : <Volume2 className="w-5 h-5" />}
                       </button>
                     </div>
-                    
-                    <div className="flex items-center space-x-2">
-                      <button
-                        onClick={handleFullscreen}
-                        className="text-white hover:text-blue-400 transition-colors duration-200"
-                        title="全屏"
-                      >
-                        <Maximize className="w-5 h-5" />
-                      </button>
-                    </div>
                   </div>
                 </div>
               </div>
             </div>
           )}
         </div>
-        
-        {/* 操作按钮 */}
-        <div className="flex items-center justify-between">
-          <div className="flex items-center space-x-2">
-            <button
-              onClick={handleDownload}
-              disabled={isDownloading}
-              className="flex items-center space-x-2 px-4 py-2 bg-green-600 hover:bg-green-700 disabled:bg-gray-400 text-white rounded-lg transition-colors duration-200"
-            >
-              <Download className="w-4 h-4" />
-              <span>{isDownloading ? '下载中...' : '下载视频'}</span>
-            </button>
-            
-            <button
-              onClick={handleOpenInNewWindow}
-              className="flex items-center space-x-2 px-4 py-2 bg-blue-600 hover:bg-blue-700 text-white rounded-lg transition-colors duration-200"
-            >
-              <ExternalLink className="w-4 h-4" />
-              <span>新窗口打开</span>
-            </button>
-          </div>
-          
-          <div className="text-sm text-gray-500">
-            {videoUrl && (
-              <span className="truncate max-w-xs" title={videoUrl}>
-                {videoUrl}
-              </span>
-            )}
-          </div>
-        </div>
       </div>
     </Modal>
   );