Viewer: render /png at 512 px like the WebSocket preview

The canvas is painted both by the fits-preview push (512 px) and by the
REST /png refresh triggered by the camera telemetry (256 px). With a
camera loop running the two alternated, and the 256 px render looked
like a frame with a coarser binning every few images.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>