feat: persist fill/fit view mode in window.json (v1.5.0)

The double-click fill/fit mode was hardcoded (image_canvas: _isFilled = true) and
reset on every image change, so it never survived a restart. It is now stored as
"filled" in window.json and restored on launch and on image switch.

WindowPersistence.saveSync() now merges with the existing file, otherwise the
startup/move/resize geometry writes would drop the mode key.

Also: version 1.5.0, CHANGELOG entry, dist/ ships imajviewer_1.5.0_amd64.deb
(replaces 1.4.2).

pubspec.yaml

@@ -1,7 +1,7 @@
 name: imajviewer
 description: "Ultra-lightweight image viewer for Linux and Windows."
 publish_to: 'none'
-version: 1.4.2+10
+version: 1.5.0+11
 
 msix_config:
   display_name: ImajViewer