v0.4.0

Index fixes, root-helper hardening, UI never freezes

- Removed roots no longer reappear after a restart.
- Elevated mode saves the index again, picks up every update, and reflects
  live changes within seconds instead of only after a full rebuild.
- Index saves fsync the containing directory too.
- Size column sort no longer drops on a new search.
- The search window no longer freezes: index loading, rebuilds, settings
  changes, elevated reloads, and Trash/Delete all run in the background.
- indexed-helper validates every directory in a path before opening it,
  refusing symlinks, planted pipes, or hard links; index files are
  validated against their declared size before parsing.

Full test suite green (332/332); clang-format and cppcheck clean.