- #150: Replace 11-parameter config_create() with config_create() that
initializes to sensible defaults; callers set fields directly
- #149: Extract validate_config() from main(); reduce main() from 325 to
281 lines by extracting validation logic into separate function
- #151: Extract run_dry_run(), connect_to_server(), send_manifest(), and
print_progress() shared helpers from send_files()/send_files_multithreaded()
to eliminate code duplication
