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>

frontend/public/sw.js

@@ -29,12 +29,17 @@ self.addEventListener('push', (event) => {
   try { data = event.data.json(); } catch {}
 
   event.waitUntil(
-    self.registration.showNotification(data.title, {
-      body: data.body,
-      icon: '/icon-192.png',
-      badge: '/icon-192.png',
-      data: { chatId: data.chatId },
-      vibrate: [200, 100, 200],
+    self.clients.matchAll({ type: 'window', includeUncontrolled: true }).then(clients => {
+      // Suppress if any app window is focused
+      const appFocused = clients.some(c => c.focused);
+      if (appFocused) return;
+      return self.registration.showNotification(data.title, {
+        body: data.body,
+        icon: '/icon-192.png',
+        badge: '/icon-192.png',
+        data: { chatId: data.chatId },
+        vibrate: [200, 100, 200],
+      });
     })
   );
 });