feat(export): implement layer export functionality as PNG images (#803)

* feat(export): implement layer export functionality as PNG images

* fix(export): await Navigator push for correct overlay navigation

* feat(video-editor): enhance export model to support audio tracks and image layers

* feat(tests): add unit tests for ExportedLayer and Layer capture functionality

* feat(release): update version to 12.1.0 and add changelog entries for layer export API and ExportedLayer model