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).

lib/widgets/image_canvas.dart

@@ -6,6 +6,7 @@ import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
 import '../l10n/app_localizations.dart';
 import '../services/crop_save_service.dart';
+import '../services/window_persistence.dart';
 
 class ImageCanvas extends StatefulWidget {
   final String filePath;
@@ -15,7 +16,7 @@ class ImageCanvas extends StatefulWidget {
 }
 
 class ImageCanvasState extends State<ImageCanvas> {
-  bool _isFilled = true;
+  bool _isFilled = WindowPersistence.filled;
   final GlobalKey _imageKey = GlobalKey();
   double _tx = 0, _ty = 0, _sc = 1.0;
   Size _vpSize = Size.zero;
@@ -199,7 +200,7 @@ class ImageCanvasState extends State<ImageCanvas> {
   }
 
   void _resetView() {
-    _tx = 0; _ty = 0; _sc = 1.0; _isFilled = true;
+    _tx = 0; _ty = 0; _sc = 1.0; _isFilled = WindowPersistence.filled;
     _angle = 0; _contrast = 1.0; _saturation = 1.0; _brightness = 0;
     _usingFullRes = false;
     _imageSize = Size.zero;
@@ -209,6 +210,7 @@ class ImageCanvasState extends State<ImageCanvas> {
   void _toggleFillFit() {
     _isFilled = !_isFilled; _tx = 0; _ty = 0; _sc = 1.0;
     _angle = 0; _contrast = 1.0; _saturation = 1.0; _brightness = 0;
+    WindowPersistence.saveFilled(_isFilled);
     _markRenderDirty();
   }