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/thingies/lib/once.d.ts

@@ -0,0 +1,5 @@
+/**
+ * A class method decorator that limits a method to be called only once. All
+ * subsequent calls will return the result of the first call.
+ */
+export declare function once<This, Args extends any[], Return>(fn: (this: This, ...args: Args) => Return, context?: ClassMethodDecoratorContext<This, (this: This, ...args: Args) => Return>): (this: This, ...args: Args) => Return;