fix: prevent double URL-rewrite in nested shard resolution

rewriteActivityUrls now skips URLs that are already absolute (start with
/ or http). Before this fix, the new user→year two-level nesting caused
year-shard URLs (/data/brut/_merged/activities/X.json) to be prepended
again at the user-shard level, producing broken doubled paths and making
every activity show "Activity not found".

site/src/lib/dataloader.ts

@@ -62,12 +62,14 @@ function isYearShardUrl(url: string): boolean {
 }
 
 function rewriteActivityUrls(a: ActivitySummary, shardBase: string): ActivitySummary {
+  // Skip if URL is already absolute (http:// or root-relative /) — avoids
+  // double-rewriting when shards are nested (e.g. user shard → year shard).
+  const needsRewrite = (url: string | null | undefined): boolean =>
+    !!url && !url.startsWith('http') && !url.startsWith('/');
   return {
     ...a,
-    detail_url: a.detail_url && !a.detail_url.startsWith('http')
-      ? `${shardBase}${a.detail_url}` : a.detail_url,
-    track_url: a.track_url && !a.track_url.startsWith('http')
-      ? `${shardBase}${a.track_url}` : a.track_url,
+    detail_url: needsRewrite(a.detail_url) ? `${shardBase}${a.detail_url}` : a.detail_url,
+    track_url:  needsRewrite(a.track_url)  ? `${shardBase}${a.track_url}`  : a.track_url,
   };
 }