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/templates/_frame_name_edit.html

@@ -7,3 +7,5 @@
   <button type="button" class="btn-inline" id="frame-name-save">Save</button>
   <button type="button" class="btn-inline secondary" id="frame-name-cancel">Cancel</button>
 </span>
+<img id="frame-preview-thumb" class="frame-preview-thumb" alt="Live preview of what the frame is displaying" title="What the frame is currently displaying -- click to refresh">
+