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.

README.md

@@ -18,14 +18,16 @@ Ultra-lightweight image viewer written in Flutter — Linux & Windows.
 - **Saturation:** Ctrl + Right drag + vertical move
 - **Brightness:** Ctrl+Shift + Right drag + vertical move
 - **Rotate:** Shift + Right drag + vertical move (~100px = 90°)
+- **Flip horizontally:** `R` — mirrors the view and clears the active effects (same reset as
+  double right click); press `R` again to undo
 - **Reset (all effects):** Double right click
 - **Navigation arrows:** bottom corners, visible on hover
 - **Settings (gear):** sort by time/name, print, shortcut info
 - **Print:** send the image to a printer (A4, fit)
 - **Save visible area:** title bar save icon or `Ctrl+S` — the on-screen region is written at
   original resolution as JPEG (quality 93) next to the source file (`photo.jpg` →
-  `photo_143025.jpg`); active effects (contrast/saturation/brightness) and rotation are baked
-  into the output; if that name already exists, nothing is written and a warning is shown
+  `photo_143025.jpg`); active effects (contrast/saturation/brightness), rotation and horizontal
+  flip are baked into the output; if that name already exists, nothing is written and a warning is shown
 - Drag & drop to open images
 - Window size/position memory (JSON persistence)
 - Set as default image viewer
@@ -86,6 +88,7 @@ folder including `imajviewer.exe` and all DLLs).
 | Ctrl + Right drag + vertical | Saturation (0.0 – 2.0) |
 | Ctrl+Shift + Right drag + vertical | Brightness (-100 – 100) |
 | Shift + Right drag + vertical | Rotate (free) |
+| R | Flip horizontally (clears effects; press again to undo) |
 | Double right click | Reset (all effects) |
 | Left / Right arrow | Previous / next image |
 | Ctrl + S | Save visible area (JPEG 93, original resolution) |