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/db/sync.ts

@@ -1,14 +1,14 @@
 import type { SQLiteDatabase } from 'expo-sqlite';
 import { getSetting, upsertRemoteActivity } from './queries';
 
-export type SyncResult = { synced: number; error?: string };
+export type SyncResult = { synced: number; total: number; error?: string };
 
 export async function syncFeed(db: SQLiteDatabase): Promise<SyncResult> {
   const instanceUrl = (await getSetting(db, 'instance_url'))?.replace(/\/$/, '');
   const token = await getSetting(db, 'api_token');
 
   if (!instanceUrl || !token) {
-    return { synced: 0, error: 'No instance configured — add one in Settings.' };
+    return { synced: 0, total: 0, error: 'No instance configured — add one in Settings.' };
   }
 
   let resp: Response;
@@ -17,14 +17,14 @@ export async function syncFeed(db: SQLiteDatabase): Promise<SyncResult> {
       headers: { Authorization: `Bearer ${token}` },
     });
   } catch {
-    return { synced: 0, error: 'Could not reach instance — check your connection.' };
+    return { synced: 0, total: 0, error: 'Could not reach instance — check your connection.' };
   }
 
   if (resp.status === 401) {
-    return { synced: 0, error: 'Session expired — reconnect in Settings.' };
+    return { synced: 0, total: 0, error: 'Session expired — reconnect in Settings.' };
   }
   if (!resp.ok) {
-    return { synced: 0, error: `Server error (${resp.status})` };
+    return { synced: 0, total: 0, error: `Server error (${resp.status})` };
   }
 
   const data: { activities?: RemoteSummary[] } = await resp.json();
@@ -48,7 +48,7 @@ export async function syncFeed(db: SQLiteDatabase): Promise<SyncResult> {
     if (changed) synced++;
   }
 
-  return { synced };
+  return { synced, total: activities.length };
 }
 
 type RemoteSummary = {