fix: handle absolute detail_url paths in loadActivity and loadTimeseries

resolveShards rewrites detail_url to absolute paths (e.g. /data/brut/_merged/activities/{id}.json)
when fetching from a user shard. loadActivity and loadTimeseries only checked for http:// prefixes
and treated /data/... paths as relative, producing double /data//data/... in the fetch URL → 404.

Fix: treat URLs starting with / as already absolute, same as http:// URLs.

site/src/lib/dataloader.ts

@@ -168,7 +168,7 @@ export async function loadActivity(
   if (cached) return cached;
 
   try {
-    const url = detailUrl.startsWith('http')
+    const url = detailUrl.startsWith('http') || detailUrl.startsWith('/')
       ? detailUrl
       : `${baseUrl}data/${detailUrl}`;
     return await fetchJSON<ActivityDetail>(url);
@@ -199,7 +199,7 @@ export async function loadTimeseries(
 
     if (timeseriesUrl.startsWith('http')) {
       url = timeseriesUrl;
-    } else if (detailUrl.startsWith('http')) {
+    } else if (detailUrl.startsWith('http') || detailUrl.startsWith('/')) {
       // absolute detailUrl (browser shard resolution) → same directory
       const dir = detailUrl.substring(0, detailUrl.lastIndexOf('/') + 1);
       url = `${dir}${filename}`;