v0.2.1 -- Fix conffile-collision bug, add clean purge

Fix a real conffile-collision bug found while explaining the packaging mechanism to a user: policies.json was being written to /distribution/policies.json, a path confirmed (via dpkg -S) to be real content LibreWolf's own package ships, and confirmed (via dpkg-query -W -f='${Conffiles}') to NOT be a registered conffile. That meant zero dpkg protection -- any routine apt upgrade of librewolf could have silently overwritten it back to stock, quietly undoing this project's hardening with no warning.

Fixed by switching to /etc/librewolf/policies/policies.json, the other real location Firefox's policy engine reads (confirmed from Mozilla's own docs), and confirmed genuinely unclaimed before the change.

Adds packaging/postrm, previously missing entirely: sudo apt purge privacyfox now correctly removes the system policies.json and any backups on purge (not plain remove), cleaning up the empty directories it created. Tested both cases in isolation.

postinst simplified as a side effect -- no longer needs LibreWolf install-dir detection, since Depends: librewolf already guarantees it's present.

Rebuilt, lintian-clean.