feat: better error messages for file permission failures

When lance-data-viewer runs as UID 1001 (the default non-root user) and mounts a volume where .lance files were written by a different UID (e.g., 1000 from an Airflow or application container), the error is:

LanceError(IO): Generic LocalFileSystem error: Unable to open file /data/imag...

The message is truncated and doesn't indicate that the root cause is a permission denial. This is a common scenario when running lance-data-viewer alongside other containers in Docker Compose.

Proposed improvements:

1. Catch permission errors explicitly and return a user-friendly message including the current UID and the file owner UID
2. Add a startup check: after connecting to DATA_PATH, attempt to read a sample file and log a warning if permissions are insufficient, rather than failing silently per-dataset
3. Add a troubleshooting note to the README about volume permissions when sharing data between containers