test: update tests and stories for imageRef, onTransform, and element accessors

Tests:
- useCropper.test: fix mock (add imageRef, getImage, getCroppedCanvas);
  add tests for getCanvas/getSelection/getImage accessors and onTransform
- Cropper.test: add tests for getSelection/getImage via ref, onTransform
  listener on image element, and grid prop rendering
- useCropperAdvanced.test: add onTransform and getImage tests; extend
  null-check test to include getImage

Stories:
- useCropperAdvanced.stories: add imageRef to all stories (required after
  breaking change); add WithOnTransform story
- Cropper.stories: fix WithEventHandlers (e.detail.bounds → e.detail);
  add WithGrid, NoGrid, and WithOnTransform stories
- useCropper.stories: add WithOnTransform story

Fix: default grid=true in Cropper component to match useCropper behaviour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>