fix: read activity shards in GET /api/feed; improve sync feedback

_merged/index.json is a shard manifest with activities:[] when the user
has >FEED_PAGE_SIZE activities. The endpoint now collects from all
index-{year}.json shard files before returning.

SyncResult gains a `total` field (activities received from server) so the
feed screen can distinguish "server returned nothing" from "all already
stored locally". Messages: "No activities on instance" / "Up to date (N)"
/ "X of N activities synced".

mobile/app/(tabs)/index.tsx

@@ -18,10 +18,12 @@ export default function FeedScreen() {
     setSyncing(false);
     if (result.error) {
       setSyncMsg(result.error);
+    } else if (result.total === 0) {
+      setSyncMsg('No activities on instance');
     } else if (result.synced === 0) {
-      setSyncMsg('Already up to date');
+      setSyncMsg(`Up to date (${result.total} activities)`);
     } else {
-      setSyncMsg(`${result.synced} ${result.synced === 1 ? 'activity' : 'activities'} synced`);
+      setSyncMsg(`${result.synced} of ${result.total} activities synced`);
     }
     setTimeout(() => setSyncMsg(null), 3500);
   }, [db]);