Fix Activate for selected printer units

[boingball]

Fixes the remaining activation failure reproduced after PR #98 merged.

Root cause

Activate copied UnitN to Unit0 through .new/.bak Rename() operations. That sequence is not reliable on all AmigaOS ENV:/ENVARC: handlers, and the UI reduced any failure to the misleading "Could not copy UnitN to Unit0" message.

Fix

• Use the selected profile already loaded in the Settings GUI.
• Write Unit0 directly through the same MODE_NEWFILE path used by Save.
• Write both ENV:MintPRINT/Unit0 and ENVARC:MintPRINT/Unit0.
• Preserve the selected unit until the operation completes, then return to Unit0 on success.
• Keep cached capability/artwork copying best-effort as before.

This should address the identical failure on both reported machines.