The backend has no built-in library default — _get_dlc_dir() returns None
when DLC_DIR is unset and config.json is unusable — so deleting DLC_DIR on
invalid-config/write-failed stranded the user with an empty library and
only a console warn (corrupt config.json, or a saved dlc_dir pointing at
an unplugged drive, previously still produced a working library via the
env fallback).
invalid-config and write-failed now export the resolved fallback as
DLC_DIR, restoring pre-#117 behaviour exactly and only where config.json
cannot own the path. Config-owned dynamic refresh is unchanged for the
configured/bootstrapped paths.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
