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.

README.adoc

@@ -11,7 +11,8 @@ The project's shape is still open — this is the initial scaffold.
 
 Early scaffold. The web application serves a vector map (MapLibre GL
 JS), with an online style and an optional offline style backed by a
-local PMTiles file, plus an animated RainViewer radar overlay.
+local PMTiles file, an animated RainViewer radar overlay, and GPX
+upload/display (points and tracks, organized in folders).
 
 == Requirements