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.

lib/services/crop_save_service.dart

@@ -17,6 +17,7 @@ class CropViewParams {
   final double vw, vh; // viewport boyutu
   final double tx, ty, sc; // pan/zoom
   final double angleDeg; // donus (derece)
+  final bool flipH; // yatay aynalama (goruntunun kendisi degil, gorunum)
   final double cx, cy; // donus merkezi (viewport ortasi)
   final double k; // kaynak -> layout olcegi
   final double ox, oy; // layout offset
@@ -29,6 +30,7 @@ class CropViewParams {
     required this.ty,
     required this.sc,
     required this.angleDeg,
+    this.flipH = false,
     required this.cx,
     required this.cy,
     required this.k,
@@ -73,6 +75,8 @@ class CropSaveService {
     final out = vp.angleDeg == 0
         ? _cropAxisAligned(source, vp)
         : _cropRotated(source, vp);
+    // Ekranda yatay aynalama acikken cikti da aynalanir (cikti = viewport'un birebir eslemesi).
+    if (vp.flipH) img.flipHorizontal(out);
     // Çıktıda yön fiziksel — EXIF tag'ini temizle (çift dönüşü önler).
     out.exif.imageIfd.orientation = null;