fix: aspect ratio distortion on image switch (v1.3.1)

Downsample used ResizeImagePolicy.exact which forced a 2048×2048
square and stretched non-square images. Switch to ResizeImagePolicy.fit
to preserve aspect ratio while still capping both dimensions.

pubspec.yaml

@@ -1,7 +1,7 @@
 name: imajviewer
 description: "Ultra-lightweight image viewer for Linux and Windows."
 publish_to: 'none'
-version: 1.3.0+5
+version: 1.3.1+6
 
 msix_config:
   display_name: ImajViewer