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.

CHANGELOG.md

@@ -4,6 +4,11 @@ All notable changes to imajViewer are documented in this file.
 
 ## [Unreleased]
 
+## [1.3.1] - 2026-08-14
+
+### Fixed
+- Aspect ratio distortion on image switch — downsample now uses `ResizeImagePolicy.fit` (was `exact`, which forced 2048×2048 square and stretched non-square images)
+
 ## [1.3.0] - 2026-08-14
 
 ### Fixed