The base import for a config with no sections now reads its file once, reports a 0-byte file as empty, treats a
missing file as the defaults, and records success or failure itself (sending the failure Event on request). The
per-config overrides that only set successful_import are removed.
wrolpi.yaml and the downloader settings import in a thread at startup (import_file_configs), and the Import, Save
and backup Import buttons run their config work in a thread.
The pre-diff import path (import_decision, record_import_version) has no callers left and is removed.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>