Ambient drying: automatically dry filament on idle printers when
humidity exceeds threshold, regardless of queue state. Separate toggle
from queue auto-drying — both can run simultaneously. Uses the same
presets, humidity threshold, and power constraint detection.
Fix: block mode (wait for drying) previously skipped the humidity
auto-stop check for already-drying printers, causing drying to
continue indefinitely. Now only prevents starting new drying.