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.

test/flip_test.dart

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+import 'dart:io';
+
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+import 'package:imajviewer/widgets/image_canvas.dart';
+
+/// Aynalama uygulandiginda render zincirinde negatif x olcekli bir Transform bulunur.
+bool _mirrored(WidgetTester tester) => tester
+    .widgetList<Transform>(find.byType(Transform))
+    .any((t) => t.transform.entry(0, 0) < 0);
+
+Future<void> _pressR(WidgetTester tester) async {
+  await tester.sendKeyDownEvent(LogicalKeyboardKey.keyR);
+  await tester.sendKeyUpEvent(LogicalKeyboardKey.keyR);
+  await tester.pump();
+  await tester.pump();
+}
+
+void main() {
+  late Directory tmp;
+  late String pngPath;
+
+  setUpAll(() {
+    tmp = Directory.systemTemp.createTempSync('flip_test');
+    pngPath = '${tmp.path}/a.png';
+    File(pngPath).writeAsBytesSync(img.encodePng(img.Image(width: 4, height: 2))!);
+  });
+
+  tearDownAll(() => tmp.deleteSync(recursive: true));
+
+  testWidgets('R yatay aynalamayi acar, tekrar basinca kapatir', (tester) async {
+    await tester.pumpWidget(MaterialApp(
+      home: SizedBox(
+        width: 400,
+        height: 300,
+        child: ImageCanvas(filePath: pngPath),
+      ),
+    ));
+    await tester.pump();
+
+    expect(_mirrored(tester), isFalse, reason: 'baslangicta aynalama olmamali');
+
+    await _pressR(tester);
+    expect(_mirrored(tester), isTrue, reason: 'R sonrasi aynalama acik olmali');
+
+    await _pressR(tester);
+    expect(_mirrored(tester), isFalse, reason: 'ikinci R sonrasi eski haline donmeli');
+  });
+}