web: answer incoming calls from the browser notification

Owner follow-up to the softphone (spec 05 §5).

- the incoming call notification comes from a service worker
  (public/sw.js) so it carries Answer and Decline buttons (Chromium);
  Answer and a plain click focus the softphone tab, Decline doesn't
- shown whenever the tab isn't focused, not only when it's hidden; kept
  up while it rings and closed once it stops
- the softphone panel says how to allow notifications when they're blocked

apps/web/public/sw.js

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
+// The softphone's incoming call notification (05 §5). Only a service worker's notifications can
+// carry buttons. The softphone tab asks for one here; Answer/Decline go back to that tab only.
+/* global self */
+self.addEventListener('message', (e) => {
+  const call = e.data?.notifyCall;
+  if (!call) return;
+  e.waitUntil(
+    self.registration.showNotification('Incoming call', {
+      body: call.body,
+      tag: call.id,
+      requireInteraction: true,
+      actions: [
+        { action: 'answer', title: 'Answer' },
+        { action: 'decline', title: 'Decline' },
+      ],
+      data: { id: call.id, tab: e.source.id },
+    }),
+  );
+});
+
+self.addEventListener('notificationclick', (e) => {
+  e.notification.close();
+  const { id, tab } = e.notification.data;
+  e.waitUntil(
+    self.clients.matchAll({ type: 'window', includeUncontrolled: true }).then(async (all) => {
+      const client = all.find((c) => c.id === tab);
+      if (!client) return;
+      if (e.action) client.postMessage({ softphone: e.action, id });
+      // Declining needs nothing from the tab; otherwise bring it back.
+      if (e.action !== 'decline') await client.focus();
+    }),
+  );
+});