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/is-plain-obj/index.js

@@ -0,0 +1,10 @@
+'use strict';
+
+module.exports = value => {
+	if (Object.prototype.toString.call(value) !== '[object Object]') {
+		return false;
+	}
+
+	const prototype = Object.getPrototypeOf(value);
+	return prototype === null || prototype === Object.prototype;
+};