Fix teccam loop stripe artifacts: use device.image for Mako

Mako's _broadcast_preview path applied viridis with percentile
auto-range, amplifying sensor hot-rows into visible stripes.
Switch to device.image (array_to_png vmin=0 vmax=255), identical
to the Mako HTTP stream pipeline. STX path unchanged.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>