[stefanodangelo]

Follow-up to the Control tab (#188), from the feedback after testing on a U1.

Target boxes (nozzle, bed). The boxes were cleared after each send, so a spinner arrow always started from an empty field and sent 1 °C. They now show the current target, so an arrow steps from the real value. While you are in a box your typed value wins over the readback, and leaving it shows the target again. An empty box still sends nothing.

Enclosure fans during a print. "Fans off" worked mid-print but the sliders were blocked, so the fans could be stopped and not started again. Stock accepts SET_PURIFIER at any time, so exhaust and inner are now allowed during a print or pause, whatever the mode (an enclosure still has to be attached). The sliders are enabled, and while a mode runs a short hint says it may override the exhaust value. Outside a print nothing changes: the sliders stay disabled while a mode runs.

Tests: both fans are accepted during a print and a pause in modes 0, 1 and 3, a missing enclosure is still refused, and the refused-while-printing cases are removed. tests/unit passes. The UI changes were checked in mock mode.

test_restart.py (root runner) is not part of this PR.

[stefanodangelo]

This PR covers your first two points on PR #188: the nozzle/bed target boxes now show the current target, and the enclosure fan sliders work during a print, with a short hint while a mode runs. The test_restart.py fixture is left to you, as discussed.