Initial commit: notification-elements-demo app

Interactive Angular 19 demo for @sda/notification-elements-ui with
6 sections: Bell & Feed, Notification Center, Inbox, Comments &
Threads, Mention Input, and Full-Featured layout. Includes mock
data, dark mode toggle, and real-time event log.

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

node_modules/piscina/test/fixtures/sleep.js

@@ -0,0 +1,12 @@
+'use strict';
+
+const { promisify } = require('util');
+const sleep = promisify(setTimeout);
+
+const buf = new Uint32Array(new SharedArrayBuffer(4));
+
+module.exports = async ({ time = 100, a }) => {
+  await sleep(time);
+  const ret = Atomics.exchange(buf, 0, a);
+  return ret;
+};