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.

CHANGELOG.md

@@ -4,6 +4,15 @@ All notable changes to imajViewer are documented in this file.
 
 ## [Unreleased]
 
+## [1.6.0] - 2026-10-14
+
+### Added
+- Horizontal flip (`R` key): mirrors the image left-right about the viewport centre and clears
+  the active effects (same reset as double right click); pressing `R` again restores the
+  original orientation. Zoom/pan are kept.
+- Crop save (`Ctrl+S`) now bakes the flip into the output as well, so the saved JPEG matches
+  the mirrored view (also in combination with rotation).
+
 ## [1.5.0] - 2026-10-12
 
 ### Added