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-inside-container/index.d.ts

@@ -0,0 +1,13 @@
+/**
+Check if the process is running inside a container.
+
+@example
+```
+import isInsideContainer from 'is-inside-container';
+
+if (isInsideContainer()) {
+	console.log('Running inside a container');
+}
+```
+*/
+export default function isInsideContainer(): boolean;