fix(settings): enforce absolute paths for all anchored upgrade branches

- misc: extend sibling_bin derivation to claude/brew/volta/bun in addition
  to npm. Return Option<String> so the anchored-path invariant cannot be
  silently violated — when bin_path has no parent directory, propagate
  None and fall back to the static command at the call site instead of
  emitting a bare command that depends on the GUI process PATH.
- misc: factor out quote_path_if_spaced to keep all five anchored
  branches consistent when the resolved path contains spaces.
- AboutSection: add preflightTools lock + try/finally around
  probeToolInstallations so fast double-clicks cannot race two probe
  rounds into concurrent executeRun calls or duplicate confirm dialogs.
- Tests: cover sibling_bin None branch and space-quoting for every
  anchored branch (claude/brew/volta/bun).