Add in-app and OS notifications for new messages

Polls every 30s for unread messages on both kid and admin sides.
Kid app shows a tappable in-page toast and fires a browser OS
notification when a parent message arrives while the tab is open.
Admin app shows a toast and OS notification when a student replies.
Clicking the OS notification focuses the tab and opens the messages view.

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

public/admin.js

@@ -72,13 +72,21 @@ async function loadAppSettings() {
   } catch {}
 }
 
+function fireOsNotif(title, body, onClick) {
+  if (!('Notification' in window) || Notification.permission !== 'granted') return;
+  const n = new Notification(title, { body, icon: '/favicon.ico' });
+  if (onClick) n.onclick = () => { window.focus(); onClick(); n.close(); };
+}
+
 async function unlock() {
   await loadAppSettings();
   $('#pin-gate').hidden = true;
   $('#console').hidden = false;
   showPanel('kids');
+  if ('Notification' in window && Notification.permission === 'default') Notification.requestPermission();
   loadGradingBadge();
   loadMessagesBadge();
+  startAdminMsgPoll();
 }
 
 async function loadGradingBadge() {
@@ -1953,6 +1961,8 @@ $('#grading-student-filter').addEventListener('change', loadGrading);
 // Messages
 // ============================================================
 
+let _adminLastMsgCount = -1;
+
 async function loadMessagesBadge() {
   try {
     const { count } = await api('/api/admin/messages/count');
@@ -1967,9 +1977,22 @@ async function loadMessagesBadge() {
       badge?.remove();
       pill.classList.remove('grading-pending');
     }
+    if (_adminLastMsgCount >= 0 && count > _adminLastMsgCount) {
+      msg(`💬 New message from a student (${count} unread)`, 'success');
+      fireOsNotif('💬 New student message', `${count} unread message${count !== 1 ? 's' : ''} waiting.`,
+        () => document.querySelector('.nav-pill[data-panel="messages"]')?.click());
+    }
+    _adminLastMsgCount = count;
   } catch { /* non-critical */ }
 }
 
+let _adminMsgPollInterval = null;
+
+function startAdminMsgPoll() {
+  clearInterval(_adminMsgPollInterval);
+  _adminMsgPollInterval = setInterval(loadMessagesBadge, 30000);
+}
+
 let activeMessageStudentId = null;
 
 async function loadMessages() {