add mention support

lib/widgets/chat_page/html/mention_chip.dart

@@ -0,0 +1,26 @@
+import "package:flutter/material.dart";
+import "package:flutter_widget_from_html_core/flutter_widget_from_html_core.dart";
+import "package:matrix/matrix.dart";
+
+class MentionChip extends StatelessWidget {
+  final String label;
+  const MentionChip(this.label, {super.key});
+
+  @override
+  Widget build(BuildContext context) => InlineCustomWidget(
+    child: ActionChip(
+      label: Text(
+        label.parseIdentifierIntoParts()?.primaryIdentifier ?? label,
+        style: TextStyle(
+          fontWeight: FontWeight.bold,
+          color: Theme.of(context).colorScheme.onPrimary,
+        ),
+      ),
+      backgroundColor: Theme.of(context).colorScheme.primary,
+      onPressed: () {
+        // TODO: Open room or join room dialog, or user popover
+        showAboutDialog(context: context);
+      },
+    ),
+  );
+}