Add a live preview thumbnail next to the frame name

Small header thumbnail showing exactly what the frame is currently
displaying -- same widget compositor /frame/image uses, handed back as
a plain PNG (image_pipeline.render_panel/render_placeholder gain an
as_png option) instead of packed native-panel bytes. New session-authed
GET /api/frames/{id}/preview exposes it; click-to-refresh plus a slow
60s poll on the frame header so it doesn't hammer Immich/calendar
sources just for a header thumbnail.

server/app/static/frame_header.js

@@ -57,3 +57,20 @@
     if (e.key === 'Escape') closeEdit();
   });
 })();
+
+// Live "how it's displaying" thumbnail. A real composite render (same
+// pipeline /frame/image uses), not a cached snapshot, so it's on a slow
+// poll and also click-to-refresh rather than something tighter like the
+// 10s device-status poll -- no need to hit Immich/calendar/whiteboard
+// sources that often just for a header thumbnail.
+(function () {
+  var thumb = document.getElementById('frame-preview-thumb');
+  if (!thumb || !window.FRAME_BASE_API) return;
+
+  function refresh() {
+    thumb.src = `${window.FRAME_BASE_API}/preview?t=${Date.now()}`;
+  }
+  thumb.addEventListener('click', refresh);
+  refresh();
+  setInterval(refresh, 60000);
+})();