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.

.gitignore

@@ -9,6 +9,10 @@
 # see docs/development.adoc)
 /data/*.pmtiles
 
+# User-uploaded GPX files
+/data/gpx/*
+!/data/gpx/.gitkeep
+
 # Editor / OS
 .DS_Store
 *.swp