fix: show total activity count in global feed counter

The counter now shows "50 of 16398 activities" using the total from
feed.json, matching the previous behaviour where all activities were
loaded upfront.

site/src/lib/dataloader.ts

@@ -269,12 +269,13 @@ interface FeedPage {
  */
 export async function loadCombinedFeed(
   baseUrl: string,
-): Promise<{ activities: ActivitySummary[]; remainingPages: number } | null> {
+): Promise<{ activities: ActivitySummary[]; remainingPages: number; totalActivities: number } | null> {
   try {
     const feed = await fetchJSON<FeedPage>(`${baseUrl}data/feed.json`);
     return {
       activities: feed.activities ?? [],
       remainingPages: (feed.total_pages ?? 1) - 1,
+      totalActivities: feed.total_activities ?? 0,
     };
   } catch {
     return null;