Display GPX traces on the trip map

[lubekpl]

The trip planner shows a Display button for a plan item's GPX, but it is disabled with a "WIP - Coming soon" tooltip, so right now an item's GPX can only be downloaded. The trip map already draws item GPX when a day is highlighted (it builds gpxData and runs it through gpxToPolyline), and the map tracker has a working per-place display, so most of the parts already exist.

I would like to enable the per-item Display button. Proposed behaviour:

• Click Display to draw the item's track and zoom to its bounds. The button changes to Hide and removes the track.
• The track clears when the item detail panel closes.
• Same behaviour in the shared (public) trip view.

While working on this I ran into two related gaps:

1. The item create endpoint does not store the gpx field, so a GPX added when creating a new item is dropped and only kept when editing an existing item afterwards. Adding gpx to the created item fixes it.
2. Selecting a place for a plan item copies the coordinates, price, name and description but not the place GPX. Pulling the place GPX onto the item (when it has none) lets a track attached to a POI reach the trip.

I have a working implementation for all three on a branch and ran it locally (the build passes and I checked the flow in the browser). Happy to open a PR if this sounds right.

Could you implement/fix it: yes.