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/postcss/lib/at-rule.js

@@ -0,0 +1,25 @@
+'use strict'
+
+let Container = require('./container')
+
+class AtRule extends Container {
+  constructor(defaults) {
+    super(defaults)
+    this.type = 'atrule'
+  }
+
+  append(...children) {
+    if (!this.proxyOf.nodes) this.nodes = []
+    return super.append(...children)
+  }
+
+  prepend(...children) {
+    if (!this.proxyOf.nodes) this.nodes = []
+    return super.prepend(...children)
+  }
+}
+
+module.exports = AtRule
+AtRule.default = AtRule
+
+Container.registerAtRule(AtRule)