Global feed: switch from sequential pages to month-based BAS shards

feed.json is now a BAS shard index pointing to feed-YYYY-MM.json files
(~150 activities / ~25 KB gzip each) instead of 400+ sequential feed-N.json
pages. The frontend can now jump directly to a specific month when filtering
by year or date range, without loading all newer data first.

- merge.py: write_combined_feed groups by YYYY-MM and emits a shard index
- dataloader.ts: isYearShardUrl matches feed-YYYY-MM.json; loadCombinedFeed
  returns pendingShards; FeedPage interface and loadCombinedFeedPage removed
- ActivityFeed.svelte: _yearFromShard handles both index-YYYY and feed-YYYY-MM;
  feedNextPage/feedTotalPages/loadingAllFeedPages removed; infinite-loop bug
  fixed (toLoad.length guard before setting loadingAllShards); onMount uses
  pendingShards from loadCombinedFeed

site/src/lib/dataloader.ts

@@ -58,7 +58,7 @@ function emptyIndex(): BASIndex {
 // ── Helpers ───────────────────────────────────────────────────────────────────
 
 function isYearShardUrl(url: string): boolean {
-  return /(?:^|\/)index-\d{4}\.json$/.test(url);
+  return /(?:^|\/)(?:index-\d{4}|feed-\d{4}-\d{2})\.json$/.test(url);
 }
 
 function rewriteActivityUrls(a: ActivitySummary, shardBase: string): ActivitySummary {
@@ -253,50 +253,28 @@ 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.
+ * Load the combined feed (multi-user global feed).
  *
- * Falls back to the full shard-resolution path if feed.json doesn't exist
- * (single-user installs, older data).
+ * feed.json is now a BAS shard index: the most-recent year shard is fetched
+ * immediately; older shards are returned as pendingShards for lazy loading.
+ * Returns null if feed.json doesn't exist (single-user installs).
  */
 export async function loadCombinedFeed(
   baseUrl: string,
-): Promise<{ activities: ActivitySummary[]; remainingPages: number; totalActivities: number } | null> {
+): Promise<{ activities: ActivitySummary[]; pendingShards: string[]; totalActivities: number } | null> {
   try {
-    const feed = await fetchJSON<FeedPage>(`${baseUrl}data/feed.json`);
+    const { index, pendingShards } = await loadIndexPaged(baseUrl, `${baseUrl}data/feed.json`);
     return {
-      activities: feed.activities ?? [],
-      remainingPages: (feed.total_pages ?? 1) - 1,
-      totalActivities: feed.total_activities ?? 0,
+      activities: index.activities,
+      pendingShards,
+      totalActivities: (index as any).total_activities ?? index.activities.length,
     };
   } 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.