Show a camera notice in Air Painting instead of a preview crash

A visitor without a camera (or who declines the permission prompt) sees the full-screen red "Preview error" overlay on /examples/air-painting, because CameraUnavailableError escapes the lil-gui Enable camera handler as an unhandled rejection. Reported by @Matt Lewis, routed by @Matias Gonzalez Fernandez.

• enableCamera now swallows only CameraUnavailableError and reports it through a new onCameraNotice renderer option; real bugs (renderer construction, model init) still surface as preview errors.
• describeCameraFailure pairs each failure reason with a short recovery hint, rendered as a role="status" card over the canvas so the visitor can fix it and press Enable camera again.
• Notice clears on every new attempt, so a successful retry removes it.