docs: document crop save feature in README (save button / Ctrl+S)

README.md

@@ -22,6 +22,10 @@ Ultra-lightweight image viewer written in Flutter — Linux & Windows.
 - **Navigation arrows:** bottom corners, visible on hover
 - **Settings (gear):** sort by time/name, print, shortcut info
 - **Print:** send the image to a printer (A4, fit)
+- **Save visible area:** title bar save icon or `Ctrl+S` — the on-screen region is written at
+  original resolution as JPEG (quality 93) next to the source file (`photo.jpg` →
+  `photo_143025.jpg`); active effects (contrast/saturation/brightness) and rotation are baked
+  into the output; if that name already exists, nothing is written and a warning is shown
 - Drag & drop to open images
 - Window size/position memory (JSON persistence)
 - Set as default image viewer
@@ -84,6 +88,7 @@ folder including `imajviewer.exe` and all DLLs).
 | Shift + Right drag + vertical | Rotate (free) |
 | Double right click | Reset (all effects) |
 | Left / Right arrow | Previous / next image |
+| Ctrl + S | Save visible area (JPEG 93, original resolution) |
 | Ctrl + Q | Close window |
 | Ctrl + Shift + Q | Close all windows |