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/widgets/image_canvas.dart

@@ -26,6 +26,7 @@ class ImageCanvasState extends State<ImageCanvas> {
   double _contrast = 1.0;
   double _saturation = 1.0;
   double _brightness = 0;
+  bool _flipH = false;
 
   // ── Gesture tracking ──
   String _interactionMode = 'none'; // 'contrast' | 'rotate' | 'saturation' | 'brightness'
@@ -165,6 +166,13 @@ class ImageCanvasState extends State<ImageCanvas> {
   bool _onHardwareKey(KeyEvent event) {
     final key = event.logicalKey;
     if (event is KeyDownEvent) {
+      if (key == LogicalKeyboardKey.keyR &&
+          !HardwareKeyboard.instance.isControlPressed &&
+          !HardwareKeyboard.instance.isShiftPressed &&
+          !HardwareKeyboard.instance.isAltPressed) {
+        _toggleFlipH();
+        return true;
+      }
       if (key == LogicalKeyboardKey.controlLeft ||
           key == LogicalKeyboardKey.controlRight) {
         _ctrlPressed = true;
@@ -202,6 +210,7 @@ class ImageCanvasState extends State<ImageCanvas> {
   void _resetView() {
     _tx = 0; _ty = 0; _sc = 1.0; _isFilled = WindowPersistence.filled;
     _angle = 0; _contrast = 1.0; _saturation = 1.0; _brightness = 0;
+    _flipH = false;
     _usingFullRes = false;
     _imageSize = Size.zero;
     _markRenderDirty();
@@ -210,10 +219,19 @@ 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;
+    _flipH = false;
     WindowPersistence.saveFilled(_isFilled);
     _markRenderDirty();
   }
 
+  /// R: sağ çift tık gibi efektleri sıfırlar ve yatay aynalar; tekrar basınca geri alır.
+  void _toggleFlipH() {
+    _flipH = !_flipH;
+    _angle = 0; _contrast = 1.0; _saturation = 1.0; _brightness = 0;
+    _interactionMode = 'none';
+    _markRenderDirty();
+  }
+
   void _handleDoubleTap() => _toggleFillFit();
 
   void _handleScroll(PointerScrollEvent e) {
@@ -261,6 +279,7 @@ class ImageCanvasState extends State<ImageCanvas> {
       _contrast = 1.0;
       _saturation = 1.0;
       _brightness = 0;
+      _flipH = false;
       _interactionMode = 'none';
       _markRenderDirty();
       _lastRightTapTime = null;
@@ -416,6 +435,7 @@ class ImageCanvasState extends State<ImageCanvas> {
       ty: _ty,
       sc: _sc,
       angleDeg: _angle,
+      flipH: _flipH,
       cx: _vpSize.width / 2,
       cy: _vpSize.height / 2,
       k: k,
@@ -518,12 +538,13 @@ class ImageCanvasState extends State<ImageCanvas> {
       },
     );
 
-    if (_angle != 0) {
+    if (_angle != 0 || _flipH) {
       imageContent = Transform(
         alignment: Alignment.topLeft,
         transform: Matrix4.identity()
           ..translate(vpCenterX, vpCenterY)
           ..rotateZ(angleRad)
+          ..scale(_flipH ? -1.0 : 1.0, 1.0)
           ..translate(-vpCenterX, -vpCenterY),
         child: imageContent,
       );