Fix push notifications: focus suppression, empty body, error visibility

- sw.js: suppress notification when the target chat tab is already focused
  (clients.matchAll visibility check before showNotification)
- generation_task.py: provide meaningful body for tool-only responses
  (lists tool names instead of sending an empty string that browsers discard);
  promote scheduling failure from debug to warning
- push.py: promote send errors from warning to error with exc_info;
  log successful sends at INFO so they're visible in normal operation

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

frontend/public/sw.js

@@ -2,12 +2,22 @@
 self.addEventListener('push', event => {
   if (!event.data) return;
   const data = event.data.json();
+  const notifUrl = data.url || '/';
+
   event.waitUntil(
-    self.registration.showNotification(data.title || 'Fabled Assistant', {
-      body: data.body || '',
-      icon: '/favicon.ico',
-      badge: '/favicon.ico',
-      data: { url: data.url || '/' },
+    clients.matchAll({ type: 'window', includeUncontrolled: true }).then(clientList => {
+      // Suppress notification if the user already has the relevant page focused
+      for (const client of clientList) {
+        if (client.visibilityState === 'visible' && client.url.includes(notifUrl)) {
+          return;
+        }
+      }
+      return self.registration.showNotification(data.title || 'Fabled Assistant', {
+        body: data.body || '',
+        icon: '/favicon.ico',
+        badge: '/favicon.ico',
+        data: { url: notifUrl },
+      });
     })
   );
 });