fix: read receipts (single/double check), push notifications for all users

- Track when others read messages via messages_read WS event
- Show Check (single) for sent, CheckCheck (double) only when read
- Send push to all recipients, SW suppresses if app is focused

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/ws.ts

@@ -27,13 +27,10 @@ async function getChatMemberIds(chatId: string): Promise<string[]> {
 }
 
 async function sendPushToOfflineUsers(uids: string[], payload: object) {
-  const online = new Set(connections.keys());
-  const offline = uids.filter(id => !online.has(id));
-  if (offline.length === 0) return;
-
+  // Send to all — SW on client side will suppress if app is in foreground
   const { rows } = await pool.query(
     'SELECT endpoint, auth_key, p256dh FROM push_subscriptions WHERE user_id = ANY($1)',
-    [offline]
+    [uids]
   );
 
   const body = JSON.stringify(payload);