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.

views/index.erb

@@ -1 +1,17 @@
 <div id="map"></div>
+
+<div id="gpx-panel" class="gpx-panel" hidden>
+  <div class="gpx-panel__header">
+    <strong>GPX</strong>
+    <button type="button" class="gpx-panel__close" aria-label="Close">&times;</button>
+  </div>
+
+  <form class="gpx-upload">
+    <input type="file" name="file" accept=".gpx" required>
+    <input type="text" name="folder" placeholder="folder/subfolder (optional)">
+    <button type="submit">Upload</button>
+    <p class="gpx-upload__status" hidden></p>
+  </form>
+
+  <div class="gpx-tree"></div>
+</div>