fix: handle absolute track_url paths in ActivityDetail

resolveShards also rewrites track_url to absolute paths (/data/…).
The trackUrl reactive statement only handled http:// prefixes,
producing double /data//data/… for the GeoJSON fetch → map had no track.

site/src/components/ActivityDetail.svelte

@@ -54,7 +54,7 @@
   }
 
   $: trackUrl = activity.track_url
-    ? (activity.track_url.startsWith('http') ? activity.track_url : `${base}data/${activity.track_url}`)
+    ? (activity.track_url.startsWith('http') || activity.track_url.startsWith('/') ? activity.track_url : `${base}data/${activity.track_url}`)
     : null;
   $: color = sportColor(activity.sport);