Add GPX upload with hierarchical folders and map display

Upload GPX files into a folder hierarchy (traversal-safe path
resolution against the store root), browse them in a tree panel, and
toggle each one's waypoints/tracks on the map. Waypoints render as
clickable points showing name, description and time; bare URLs in the
description are turned into safe links. GPX layers survive switching
between the Online/Offline base map, same as the radar overlay.

Adds rexml as an explicit dependency — it moved to a bundled gem in
this Ruby version and isn't picked up by Bundler otherwise.

public/js/map.js

@@ -4,6 +4,7 @@ import {
   addProtocol,
 } from "/vendor/maplibre-gl/maplibre-gl.mjs";
 import { RadarControl } from "/js/radar.js";
+import { GpxPanelControl } from "/js/gpx.js";
 
 const CENTER = [17.1077, 48.1486];
 const ZOOM = 13;
@@ -74,6 +75,7 @@ map.addControl(new RadarControl(map), "top-right");
 const layerToggle = new LayerToggleControl(applyStyle);
 map.addControl(layerToggle, "top-left");
 layerToggle.setActive("online");
+map.addControl(new GpxPanelControl(map), "top-left");
 
 function applyStyle(name) {
   layerToggle.setActive(name);