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.js

@@ -0,0 +1,19 @@
+"use strict";
+/* tslint:disable no-invalid-this */
+Object.defineProperty(exports, "__esModule", { value: true });
+exports.once = once;
+const instances = new WeakMap();
+/**
+ * A class method decorator that limits a method to be called only once. All
+ * subsequent calls will return the result of the first call.
+ */
+function once(fn, context) {
+    return function (...args) {
+        let map = instances.get(this);
+        if (!map)
+            instances.set(this, (map = new WeakMap()));
+        if (!map.has(fn))
+            map.set(fn, fn.apply(this, args));
+        return map.get(fn);
+    };
+}