Extract uploadImageFile to shared hook, add useImageDrop hook that
handles dragover/dragleave/drop events on the editor container.
Drops image files (jpg, png, gif, webp), uploads them via the existing
save_image flow, and inserts BlockNote image blocks at the drop position.
Visual feedback via drop overlay and dashed border.
Refactored Editor.tsx uploadFile to use the shared function.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
8a5df707f5