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/autoprefixer/lib/hacks/placeholder-shown.js

@@ -0,0 +1,17 @@
+let Selector = require('../selector')
+
+class PlaceholderShown extends Selector {
+  /**
+   * Return different selectors depend on prefix
+   */
+  prefixed(prefix) {
+    if (prefix === '-ms-') {
+      return ':-ms-input-placeholder'
+    }
+    return `:${prefix}placeholder-shown`
+  }
+}
+
+PlaceholderShown.names = [':placeholder-shown']
+
+module.exports = PlaceholderShown