The server now records exactly what was last sent to the device on
every device-facing render (/frame/image, /frame/advance, /frame/back,
and the global hold actions), persisted as Frame.last_displayed_image/
_at and served back via GET /api/frames/{id}/now-displaying. The
header thumbnail is split into that frozen "now displaying" snapshot
and the existing live "up next" re-render, with an arrow between them
-- so editing a layout shows the change immediately on the right while
the left stays exactly what's actually on the panel until the device's
next real wake.
