add folder depth to backend and frontend.

[cudmore]

I want to expand searching for tif files in a folder to include a folder depth.

1. Right now in kym_file.py we have collect_metadata() as:

"""
def collect_metadata(root: str | Path, **kwargs: Any) -> List[Dict[str, Any]]:
"""Collect metadata for all TIFF files under a root directory.
"""

Expand this to take a param like:
folder_depth:int = 1

Folder depth of 1 will be the same as our current behavior.

2. add this concept to the nicegui interface in src/kymflow_gui

• add a nicegui int setter 'Folder Depth' in same row as 'load folder'. The value of this will be passed to collect_metadata() as folder_depth

Summary

I assume this will just populate our gui file_table.py with a long list of tif files (including subfolders).