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/domain/document/operations/mod.rs

@@ -3,10 +3,14 @@
 //
 // Document operations: transformations, rendering, and export.
 
+pub mod crop;
 pub mod export;
 pub mod render;
 pub mod transform;
 
+// Re-export CropRegion for convenience
+pub use crop::CropRegion;
+
 // Note: Low-level pixel operations (apply_rotation, apply_flip, crop_image)
 // are internal helpers (pub(crate)) used only by document type implementations.
 // Use high-level operations above for application and UI code.