Show clicked hiking trail info in a docked panel, not a popup

A map-anchored Popup on a trail click routinely overlapped the very
line it described. Follows the same click -> docked panel pattern
gpx.js already uses for a track line click (TrackProfilePanel) rather
than inventing a new one: a fixed #trail-panel docks to the right edge
on desktop and the bottom edge on narrow viewports (this file's first
@media breakpoint), so the trail stays fully visible either way.

routeHtml()'s own markup is reused unchanged for the panel body —
.trail-panel__body's padding matches .maplibregl-popup-content's own
(15px 10px) so the header's canceling negative margin still lines up.
The popup-identity staleness guard for the async Overpass lookup is
replaced with a plain request token, since there's no longer a fresh
Popup instance to key off of.

views/index.erb

@@ -31,6 +31,11 @@
   <div class="profile-panel__charts"></div>
 </div>
 
+<div id="trail-panel" class="trail-panel" hidden>
+  <button type="button" class="trail-panel__close" aria-label="Close">&times;</button>
+  <div class="trail-panel__body"></div>
+</div>
+
 <div id="basemap-panel" class="basemap-panel" hidden>
   <div class="basemap-panel__header">
     <strong>Base map</strong>