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).

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to imajViewer are documented in this file.
 
 ## [Unreleased]
 
+## [1.5.0] - 2026-10-12
+
+### Added
+- Fill/fit mode (double-click) is now persisted in `window.json` and restored on the next launch
+  and when switching images, instead of always resetting to fill.
+
 ## [1.4.2] - 2026-10-12
 
 ### Fixed