feat: horizontal flip with R key, baked into crop save (v1.6.0)

- R mirrors the view about the viewport centre and clears the active effects
  (same reset as double right click); pressing R again restores the original
  orientation. Pan/zoom are kept.
- Image switch, fill/fit toggle and right double click also clear the flip.
- Crop save (Ctrl+S) mirrors the output too, so the saved JPEG matches the
  on-screen view (verified in combination with rotation).
- Shortcut lists (tr/en), README and CHANGELOG updated; deb rebuilt as 1.6.0.

.gitignore

@@ -49,7 +49,7 @@ app.*.map.json
 *.deb
 *.tar.gz
 *.exe
-!dist/imajviewer_1.5.0_amd64.deb
+!dist/imajviewer_1.6.0_amd64.deb
 !dist/imajviewer_setup_1.1.0.exe
 deb/usr/lib/imajviewer/
 release/