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.d.ts

@@ -0,0 +1,29 @@
+/**
+Check if a value is a plain object.
+
+An object is plain if it's created by either `{}`, `new Object()`, or `Object.create(null)`.
+
+@example
+```
+import isPlainObject = require('is-plain-obj');
+
+isPlainObject({foo: 'bar'});
+//=> true
+
+isPlainObject(new Object());
+//=> true
+
+isPlainObject(Object.create(null));
+//=> true
+
+isPlainObject([1, 2, 3]);
+//=> false
+
+class Unicorn {}
+isPlainObject(new Unicorn());
+//=> false
+```
+*/
+declare function isPlainObject<Value = unknown>(value: unknown): value is Record<string | number | symbol, Value>;
+
+export = isPlainObject;