fix(ui): preserve aspect ratio in thumbnail image display

Change fillMode from PreserveAspectCrop to PreserveAspectFit to ensure
thumbnail images maintain their original aspect ratio without cropping,
preventing distortion of image content in the thumbnail delegate.

log: preserve aspect ratio in thumbnail image display
bug: https://pms.uniontech.com/bug-view-346079.html

Author: add-uos

src/qml/ThumbnailDelegate/ThumbnailImage.qml

@@ -30,7 +30,7 @@ Item {
         anchors.centerIn: thumbnailImage
         asynchronous: true
         cache: false
-        fillMode: Image.PreserveAspectCrop
+        fillMode: Image.PreserveAspectFit
         height: thumbnailImage.height
         // 用于在旋转过程中不显示白边，图片大小比例 1 -> sqrt(2) -> 1
         scale: {