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/l10n/app_en.arb

@@ -6,10 +6,10 @@
   "sortByName": "By name",
   "appDescription": "Ultra-lightweight and fast image viewer. Open images and use arrows or keyboard keys to navigate.",
   "shortcuts": "Shortcuts",
-  "shortcutsList": "• Left/Right arrow — previous/next image\n• Ctrl+S — save visible area (JPEG 90)\n• Ctrl+Q — close window\n• Ctrl+Shift+Q — close all windows\n• Double click — toggle fill/fit\n• Right-drag — contrast\n• Shift+right-drag — rotate\n• Ctrl+right-drag — saturation\n• Ctrl+Shift+right-drag — brightness",
+  "shortcutsList": "• Left/Right arrow — previous/next image\n• Ctrl+S — save visible area (JPEG 93)\n• Ctrl+Q — close window\n• Ctrl+Shift+Q — close all windows\n• Double click — toggle fill/fit\n• Right-drag — contrast\n• Shift+right-drag — rotate\n• Ctrl+right-drag — saturation\n• Ctrl+Shift+right-drag — brightness",
   "print": "Print",
   "close": "Close",
-  "saveCrop": "Save visible area (JPEG 90)",
+  "saveCrop": "Save visible area (JPEG 93)",
   "cropSaved": "Saved: {path}",
   "@cropSaved": {
     "placeholders": {