Remove obsolete 'critical' toggle from device priority card (#65)

The 'critical' flag is superseded by control_mode: off which already
prevents SEM from touching a device. The critical toggle added confusion
by overlapping with control_mode.

Removed from UI only — backend is_critical checks kept for backwards
compatibility (harmless, devices with control_mode: off are already
filtered from shedding).

Closes #65

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: belinea4071