fix(pwa): N105 — notification clicks work after the tab is closed

OS notifications were shown via page-level `new Notification()` whose onclick
only works while the originating tab is alive — clicking a notification after
closing the tab did nothing.

- New `showOsNotification()` (utils/dom) prefers `registration.showNotification()`
  so the notification is service-worker-owned and persists; falls back to
  `new Notification()` (with the previous onclick) when no SW is available, so
  worst case is unchanged behaviour.
- sw.ts gains a `notificationclick` handler: focuses an existing app window and
  forwards the target path, or opens the app if none is open.
- ClientNonUIFeatures forwards the SW `notificationClick` message to react-router
  `navigate()` (works for both hash and browser router configs), and uses a
  per-room `tag` to coalesce notifications (replacing the old notifRef.close()
  dedup a SW notification can't hold).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/dom.ts

@@ -254,6 +254,42 @@ export const notificationPermission = (permission: NotificationPermission) => {
   return true;
 };
 
+/**
+ * Show an OS notification.
+ *
+ * Prefers a service-worker-owned notification (`registration.showNotification`)
+ * so the notification persists and its click is handled by the SW
+ * `notificationclick` listener even after the originating tab is closed — the
+ * data.path is forwarded to the SW. Falls back to a page-level `Notification`
+ * (with the provided `onClick`) when no service worker is available, preserving
+ * the previous behaviour.
+ */
+export const showOsNotification = async (
+  title: string,
+  options: NotificationOptions & { data?: { path?: string } },
+  onClick?: () => void,
+): Promise<void> => {
+  try {
+    if ('serviceWorker' in navigator) {
+      const registration = await navigator.serviceWorker.ready;
+      if (registration && typeof registration.showNotification === 'function') {
+        await registration.showNotification(title, options);
+        return;
+      }
+    }
+  } catch {
+    // fall through to the page-level Notification below
+  }
+
+  const noti = new Notification(title, options);
+  if (onClick) {
+    noti.onclick = () => {
+      onClick();
+      noti.close();
+    };
+  }
+};
+
 export const getMouseEventCords = (event: MouseEvent) => ({
   x: event.clientX,
   y: event.clientY,