Grow stills at 1920×1080, cropped to the camera's region and colour-corrected (split from #584) #589

Split from #584 (its done-when 2 and 3). #584's pull keeps a look with each camera: look.region (fractions of the frame) and look.gains (gray-world or white-card), set on /admin → Cameras → Tune. The pure maths is cropRect, applyGains and visibility in Software/so101/src/ui/still-frame.js.

Done when:

1. #583's stills are grabbed at 1920×1080 where the camera supports it, and the stream stays 640×360. ustreamer holds the device, so the README says how: for example a second, low-rate ustreamer sink, or a short pause of the stream.
2. Each still is cropped with cropRect(look.region, 1920, 1080). When the camera has gains, a corrected copy is kept beside the raw one and labelled "corrected".
3. Optional, as #584 said: if #582's Switch has a white photo light, it is on for 2 s around each still, through the schedule's driver only, never by cutting USB.
4. Tests: a fixture still is cropped and corrected; the raw one is kept as it was.