ActivityDetail: fetch timeseries in parallel with detail JSON to cut load time

site/src/components/ActivityDetail.svelte

@@ -52,15 +52,27 @@
       .then(d => { segmentEfforts = d; })
       .catch(() => {});
 
+    // Start timeseries fetch immediately in parallel with detail — the URL is
+    // predictable from detail_url, saving a full sequential round-trip.
+    const detailUrl = activity.detail_url ?? '';
+    const earlyTsPromise: Promise<Timeseries | null> = detailUrl && !detailUrl.startsWith('idb:')
+      ? loadTimeseries(
+          detailUrl.replace(/\.json$/, '.timeseries.json').replace(/^.*activities\//, 'activities/'),
+          detailUrl,
+          base,
+        ).catch(() => null)
+      : Promise.resolve(null);
+
     try {
-      detail = await loadActivity(activity.id, activity.detail_url ?? '', base);
+      detail = await loadActivity(activity.id, detailUrl, base);
       if (!detail) throw new Error('Activity not found');
-      // Use embedded timeseries (IDB activities) or lazy-fetch from URL
+      // Use embedded timeseries (IDB activities) or the already-in-flight fetch
       if (detail.timeseries) {
         timeseries = detail.timeseries;
       } else if (detail.timeseries_url) {
         timeseriesLoading = true;
-        timeseries = await loadTimeseries(detail.timeseries_url, activity.detail_url ?? '', base);
+        timeseries = await earlyTsPromise ??
+          await loadTimeseries(detail.timeseries_url, detailUrl, base);
         timeseriesLoading = false;
       }
     } catch (e: any) {