Grow card: "Camera can't see the plants" hint when the camera's region is mostly blown out or black (split from #584) #590

Split from #584 (its done-when 4). #584's pull adds visibility(pixels, width, rect) and CANT_SEE in Software/so101/src/ui/still-frame.js. They already run on the Tune panel (/admin → Cameras → Tune), over the region the owner draws (look.region).

Done when:

1. #581's grow card runs visibility on its camera's latest frame or still, inside look.region. When the verdict is not "ok", the card shows: "Camera can't see the plants: move it inside or beside the box, below the light." with a link to Tune.
2. Tests: an overexposed fixture shows the hint on the rendered card, and a fine one does not.