fix: skip permission denied errors in recursive_directory_iterator

[schirik]

Summary

This PR prevents sdcpp-webui from error Service Unavailable when scanning directories with restricted system folders (like lost+found) by adding fs::directory_options::skip_permission_denied to the recursive_directory_iterator.

Additional Information

When lora_model_dir defaults to the current working directory (.) and the server is executed from specific directory, fs::recursive_directory_iterator attempts to scan system folders like lost+found.

Since regular users lack read permissions for these directories, the iterator throws a fatal std::filesystem::filesystem_error (Permission denied), resulting in an EXCEPTION_WHAT crash and partially breaking the web server routing (leading to Service Unavailable errors in the UI).

How to reproduce

1. cd /
2. Run sd-server as a non-root user
3. Observe the Service Unavailable in web (filesystem error: cannot increment recursive directory iterator: Permission denied crash upon UI initialization).

[leejet]

Thank you for your contribution.