Add "now displaying" / "up next" preview pair to the frame header

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.

server/app/routers/api_frames.py

@@ -271,6 +271,25 @@ def api_frame_preview(
     return Response(content=png, media_type="image/png")
 
 
+@router.get("/api/frames/{frame_id}/now-displaying")
+def api_frame_now_displaying(frame: Frame = Depends(require_frame_view)):
+    """Exactly what was last actually sent to this frame's device (see
+    routers/device.py's _record_last_displayed) -- the frozen "now
+    displaying" half of the header preview pair, as opposed to /preview's
+    always-live "up next" re-render. 404 (not a placeholder image) until
+    the device has fetched at least once, so the web UI can show its own
+    empty state instead of a broken image. X-Displayed-At carries the
+    capture time (unix seconds) for a "N ago" label -- a header, not the
+    body, since the body is the raw PNG bytes."""
+    if frame.last_displayed_image is None:
+        raise HTTPException(404, "This frame hasn't displayed anything yet")
+    return Response(
+        content=frame.last_displayed_image,
+        media_type="image/png",
+        headers={"X-Displayed-At": str(frame.last_displayed_at)},
+    )
+
+
 @router.get("/api/frames/{frame_id}/battery-log")
 def api_battery_log(frame: Frame = Depends(require_frame_view), db: Session = Depends(get_db)):
     rows = db.execute(