Preserve saved Unit profiles when activating

[boingball]

Fixes the regression exposed after PR #99 and closes the remaining activation issue from #75.

What changes

• Keep Unit0 through Unit7 as independent saved printer profiles.
• Add a separate live profile:
  • ENV:MintPRINT/Active for the current runtime
  • ENVARC:MintPRINT/Active as the persistent copy
• Record the selected active profile in ENV:MintPRINT/ActiveUnit and ENVARC:MintPRINT/ActiveUnit so Settings reopens on the active unit.
• Make the driver prefer Active, with the old ENV:/ENVARC:MintPRINT/Unit0 paths retained as a backward-compatible fallback.
• Activate writes the selected profile to Active and never overwrites any UnitN file.
• Saving the active Unit updates both its saved profile and the live Active copy.
• Bump the Settings/release documentation to 1.3.3.

This preserves the existing config format and printer.device job path while separating live printer state from saved profiles.