feat: crop save (save visible area at original resolution) v1.4.0

- Save visible area at original resolution as JPEG q90 (title bar button or Ctrl+S)
- Output name: original name + _HHmmss suffix; no overwrite, warning shown
- Active color effects baked in; rotation saves bounding box
- New: crop_save_service, l10n strings TR/EN, docs/plan-crop-save.md

lib/l10n/app_en.arb

@@ -6,9 +6,19 @@
   "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+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 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",
   "print": "Print",
   "close": "Close",
+  "saveCrop": "Save visible area (JPEG 90)",
+  "cropSaved": "Saved: {path}",
+  "@cropSaved": {
+    "placeholders": {
+      "path": { "type": "String" }
+    }
+  },
+  "cropFileExists": "A file with the same name already exists — nothing was written",
+  "cropUnsupported": "This image format cannot be saved as a crop",
+  "cropFailed": "Failed to save crop",
   "dragDropHint": "Drag images here\nor press Ctrl+O to open",
   "dropHere": "Drop",
   "imageLoadError": "Failed to load image"