Fix a severe GPX parse slowdown and add route (<rte>) support

GPX tracks were silently failing to render for large real-world files:
point_extensions used REXML's "extensions//*" XPath, and the // descendant
axis costs low-single-digit milliseconds per call regardless of subtree
size. Run once per point, that's negligible on small files but ~30s on a
real 8000-point track — long enough to hit a proxy timeout in production,
so the file just never appeared with no error. Replaced with plain
recursive child traversal (~40x faster on the same file, confirmed
29.9s -> 3.16s); test_parsing_many_points_with_extensions_stays_fast
guards against reintroducing it.

Also adds support for <rte> routes (not just <trk> tracks and <wpt>
waypoints), sharing the track/route conversion logic via a new
line_feature helper. Routes get their own "route" kind and a dashed
ROUTE_ICON in the file tree so the type is visible at a glance.

public/js/gpx.js

@@ -1,5 +1,5 @@
 import { Popup } from "/vendor/maplibre-gl/maplibre-gl.mjs";
-import { GPX_ICON, NEW_FOLDER_ICON, UPLOAD_ICON, WAYPOINT_ICON, TRACK_ICON } from "/js/icons.js";
+import { GPX_ICON, NEW_FOLDER_ICON, UPLOAD_ICON, WAYPOINT_ICON, TRACK_ICON, ROUTE_ICON } from "/js/icons.js";
 import { notifySidePanelOpen, onlyOneSidePanelOpen } from "/js/panels.js";
 import { formatActivityType } from "/js/track_profile.js";
 
@@ -226,6 +226,9 @@ export class GpxPanelControl {
     if (kind === "waypoint") {
       icon.innerHTML = WAYPOINT_ICON;
       icon.title = "Has waypoints";
+    } else if (kind === "route") {
+      icon.innerHTML = ROUTE_ICON;
+      icon.title = "Has a route";
     } else {
       icon.innerHTML = TRACK_ICON;
       icon.title = "Has a track";