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/css/style.css

@@ -582,6 +582,21 @@ html, body {
   line-height: 1.4;
 }
 
+.gpx-popup__photos {
+  display: flex;
+  gap: 4px;
+  margin: 4px 0;
+  overflow-x: auto;
+}
+
+.gpx-popup__photo {
+  width: 64px;
+  height: 64px;
+  flex-shrink: 0;
+  object-fit: cover;
+  border-radius: 4px;
+}
+
 .gpx-popup__meta {
   margin-top: 8px;
   padding-top: 6px;