fix: url-gen imports, overlay rules, and upload widget race condition

- .cursorrules: correct @cloudinary/url-gen paths (text/image from qualifiers/source, Position from qualifiers/position); canonical overlay block and table as single source of truth in rules; golden rule; common error for wrong overlay imports
- UploadWidget: wait for createUploadWidget (poll when script in page, onload when injecting); fix click listener cleanup with ref

Author: strausr