perf: combined feed index for multi-user global feed

Instead of the browser resolving 20+ user shards recursively (~27 MB),
generate a pre-sorted feed.json at merge time with 50 activities per
page. The global feed loads one ~30 KB file on first paint; "Load more"
fetches subsequent pages (feed-2.json, feed-3.json, etc.).

Per-user profile pages still use year-sharded loadIndexPaged as before.

site/src/lib/dataloader.ts

@@ -253,6 +253,49 @@ export async function loadShardActivities(shardUrl: string): Promise<ActivitySum
   }
 }
 
+interface FeedPage {
+  page: number;
+  total_pages: number;
+  total_activities: number;
+  activities: ActivitySummary[];
+}
+
+/**
+ * Load the combined feed (multi-user global feed). Returns the first page of
+ * activities pre-sorted across all users, plus remaining page count.
+ *
+ * Falls back to the full shard-resolution path if feed.json doesn't exist
+ * (single-user installs, older data).
+ */
+export async function loadCombinedFeed(
+  baseUrl: string,
+): Promise<{ activities: ActivitySummary[]; remainingPages: number } | null> {
+  try {
+    const feed = await fetchJSON<FeedPage>(`${baseUrl}data/feed.json`);
+    return {
+      activities: feed.activities ?? [],
+      remainingPages: (feed.total_pages ?? 1) - 1,
+    };
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Load a subsequent page of the combined feed (feed-2.json, feed-3.json, etc.).
+ */
+export async function loadCombinedFeedPage(
+  baseUrl: string,
+  page: number,
+): Promise<ActivitySummary[]> {
+  try {
+    const feed = await fetchJSON<FeedPage>(`${baseUrl}data/feed-${page}.json`);
+    return feed.activities ?? [];
+  } catch {
+    return [];
+  }
+}
+
 /**
  * Load a single activity detail, checking IndexedDB first so locally-converted
  * activities are available offline.