fix: image_canvas correctness + RAM fixes (v1.3.0)

- _imageSize reset on image switch (wrong swap threshold on nav)
- downsample caps both dims (tall images no longer full-res decode)
- single _readImageSize instead of leaking loadingBuilder listeners
- evict old downsample provider on full-res swap
- remove imageCache.clear() that dropped live image (gallery flash)

pubspec.yaml

@@ -1,7 +1,7 @@
 name: imajviewer
 description: "Ultra-lightweight image viewer for Linux and Windows."
 publish_to: 'none'
-version: 1.2.0+4
+version: 1.3.0+5
 
 msix_config:
   display_name: ImajViewer