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-what/test/index.test.js

@@ -0,0 +1,15 @@
+// @ts-check
+import {
+  isBlob,
+  isFile,
+} from '../dist/index.cjs'
+
+test('isBlob', () => {
+  expect(isBlob(Blob)).toBe(false)
+  expect(isBlob(new Blob())).toBe(true)
+})
+
+test('isFile', () => {
+  expect(isFile(File)).toBe(false)
+  expect(isFile(new File([''], '', { type: 'text/html' }))).toBe(true)
+})