fix: crop save always JPEG q93 + save rotated view 1:1 (v1.4.1)

- Output extension is always .jpg regardless of source format (was: PNG in -> PNG out)
- Rotation now saved correctly: viewport grid resampled bilinearly in source
  space (was: unrotated bounding box copy)
- Quality 90 -> 93; l10n strings updated (JPEG 93)
- New: tool/crop_test.dart — pure-Dart end-to-end test (dart run tool/crop_test.dart)

lib/screens/viewer_screen.dart

@@ -198,14 +198,14 @@ class _ViewerScreenState extends State<ViewerScreen> with WindowListener {
     String? message;
     try {
       final source = await CropSaveService.load(path);
-      final rect = state.cropSourceRect(source.width, source.height);
-      if (rect == null || rect.isEmpty || !rect.isFinite) {
+      final vp = state.cropViewParams(source.width, source.height);
+      if (vp == null) {
         message = l10n.cropFailed;
       } else {
         final outPath = await CropSaveService.saveCrop(
           srcPath: path,
           source: source,
-          rect: rect,
+          vp: vp,
           matrix: state.activeColorMatrix,
         );
         message = l10n.cropSaved(outPath);