Setup stopped on 'HOME: unbound variable' when neither HOME nor
XDG_DATA_HOME was set (or XDG_DATA_HOME was relative without HOME).
Expand HOME defensively like the install lock already does and fail
with the missing data-directory cause instead.

Docs: no guide changes; the setup command, help text, and installer
flow are unchanged, and installer messages stay technical English
outside the QML catalogs. TODO is unaffected (not a parity gap).