Show waypoint photos from newer p4n2gpx exports

A newer p4n2gpx run (Ruby rewrite, ~/test/mapping/p4n2gpx) started
emitting one plain GPX <link href> per park4night listing photo,
which this app parsed but never displayed. Gpx.waypoint_photos
collects them into a photos property; the waypoint popup renders them
as a row of thumbnails above the description, each opening full-size
in a new tab. Absent on older exports and plain GPS waypoints, so the
row just doesn't render for those.

public/js/gpx.js

@@ -754,10 +754,11 @@ function gpxFileUrl(path) {
 
 // properties come straight from an uploaded GPX file — untrusted input —
 // so every value is escaped before landing in the popup's HTML.
-function popupHtml({ name, description, time, category }) {
+function popupHtml({ name, description, time, category, photos }) {
   const parts = [];
   if (name) parts.push(`<strong>${escapeHtml(name)}</strong>`);
   if (category) parts.push(`<p class="gpx-popup__category">${escapeHtml(category)}</p>`);
+  if (photos?.length > 0) parts.push(formatPhotos(photos));
   if (description) parts.push(formatDescription(description));
   if (time) {
     const parsed = new Date(time);
@@ -808,6 +809,22 @@ function escapeHtml(text) {
 const DESCRIPTION_META_LINE = /^(Rating|Services|Activities):\s*(.+)$/i;
 const DESCRIPTION_LINK_LINE = /^https?:\/\/\S+$/;
 
+// <link> photo URLs (a newer p4n2gpx export adds one per park4night listing
+// photo — see lib/maptool/gpx.rb's waypoint_photos) as a row of thumbnails,
+// each opening the full-size image in a new tab. Untrusted URLs from an
+// uploaded file, same as everywhere else in this popup — escaped for use as
+// both an href and an img src.
+function formatPhotos(photos) {
+  const thumbs = photos
+    .map((url) => escapeHtml(url))
+    .map(
+      (url) =>
+        `<a href="${url}" target="_blank" rel="noopener noreferrer"><img class="gpx-popup__photo" src="${url}" loading="lazy" alt=""></a>`,
+    )
+    .join("");
+  return `<div class="gpx-popup__photos">${thumbs}</div>`;
+}
+
 function formatDescription(description) {
   const body = [];
   const meta = [];