refactor: Clean Architecture - Separate domain (CropRegion) from UI (crop_model)

- CropRegion moved to domain/document/operations/crop.rs (pure model)
- crop_types.rs renamed to crop_model.rs (honest UI model name)
- DragHandle and CropSelection stay in UI layer (where they belong)
- Clean separation: Domain has no UI concerns, UI imports from Domain

src/ui/widgets/crop_overlay.rs

@@ -18,7 +18,7 @@ use cosmic::{
     },
 };
 
-use crate::ui::widgets::{CropSelection, DragHandle};
+use crate::ui::widgets::crop_model::{CropSelection, DragHandle};
 use crate::ui::AppMessage;
 
 // Visual constants