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/css/style.css

@@ -76,3 +76,120 @@ html, body {
   font-variant-numeric: tabular-nums;
   white-space: nowrap;
 }
+
+.gpx-toggle button {
+  /* Override maplibregl-ctrl-group's 29x29px square icon-button sizing —
+     this is a text button. */
+  width: auto;
+  height: auto;
+  border: none;
+  background: #fff;
+  padding: 6px 10px;
+  font: inherit;
+  font-size: 13px;
+  white-space: nowrap;
+  cursor: pointer;
+}
+
+.gpx-panel {
+  position: fixed;
+  top: 0;
+  right: 0;
+  width: 300px;
+  max-width: 90vw;
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+  background: #fff;
+  box-shadow: -2px 0 8px rgba(0, 0, 0, 0.15);
+  font-size: 13px;
+  z-index: 10;
+}
+
+.gpx-panel[hidden] {
+  display: none;
+}
+
+.gpx-panel__header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 10px 12px;
+  border-bottom: 1px solid #eee;
+}
+
+.gpx-panel__close {
+  border: none;
+  background: none;
+  font-size: 18px;
+  line-height: 1;
+  padding: 0 4px;
+  cursor: pointer;
+}
+
+.gpx-upload {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  padding: 10px 12px;
+  border-bottom: 1px solid #eee;
+}
+
+.gpx-upload input {
+  font: inherit;
+  font-size: 13px;
+}
+
+.gpx-upload button {
+  align-self: flex-start;
+  font: inherit;
+  font-size: 13px;
+  cursor: pointer;
+}
+
+.gpx-upload__status {
+  margin: 0;
+  color: #666;
+}
+
+.gpx-tree {
+  overflow-y: auto;
+  padding: 8px 12px;
+}
+
+.gpx-tree ul {
+  list-style: none;
+  margin: 0;
+  padding-left: 16px;
+}
+
+.gpx-tree > ul {
+  padding-left: 0;
+}
+
+.gpx-tree summary {
+  cursor: pointer;
+  font-weight: 600;
+}
+
+.gpx-tree__file {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  padding: 2px 0;
+  cursor: pointer;
+}
+
+.gpx-tree__empty {
+  color: #888;
+  font-style: italic;
+}
+
+.maplibregl-popup-content p {
+  margin: 4px 0 0;
+}
+
+.gpx-popup__time {
+  color: #888;
+  font-size: 12px;
+}