Canvas: Filter image fields to string type only

The image field picker currently allows selecting any field type which causes runtime errors since only string fields are valid for image paths. Add a field type filter to only show `FieldType.string` fields in the field picker dropdown. Take into account what users have currently selected. <img width="895" height="586" alt="Image" src="https://github.com/user-attachments/assets/07750610-afc2-4e6e-b3fe-8b64d1ca2b76" />