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/exponential-backoff/dist/backoff.d.ts

@@ -0,0 +1,9 @@
+import { IBackOffOptions, BackoffOptions } from "./options";
+export { BackoffOptions, IBackOffOptions };
+/**
+ * Executes a function with exponential backoff.
+ * @param request the function to be executed
+ * @param options options to customize the backoff behavior
+ * @returns Promise that resolves to the result of the `request` function
+ */
+export declare function backOff<T>(request: () => Promise<T>, options?: BackoffOptions): Promise<T>;