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.

lib/widgets/image_canvas.dart

@@ -426,7 +426,12 @@ class _ImageCanvasState extends State<ImageCanvas> {
       _cachedCacheWidth = effectiveWidth;
       _cachedFileImage = FileImage(File(widget.filePath));
       _cachedProvider = effectiveWidth != null
-          ? ResizeImage.resizeIfNeeded(effectiveWidth, effectiveWidth, _cachedFileImage!)
+          ? ResizeImage(
+              _cachedFileImage!,
+              width: effectiveWidth,
+              height: effectiveWidth,
+              policy: ResizeImagePolicy.fit,
+            )
           : _cachedFileImage;
       _readImageSize(_cachedProvider!);
     }