Collapse scanner-card and pagination duplication into shared widgets

Extract ScannerStatusCard<T> and PaginationBar so the ARP/mDNS cards and
the two paginated lists share one implementation each. Also fold the
four UISnackbars methods over a severity enum and fix Notification.toJson
serializing a method tear-off instead of the enum name.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/lib/model/notification.dart

@@ -43,7 +43,7 @@ class Notification {
   Map<String, dynamic> toJson() => {
     'id': id,
     'created_on': createdOn.toUtc().toIso8601String(),
-    'notification_type': notificationType.toString,
+    'notification_type': notificationType.name,
     'title': title,
     'body': body,
     'is_new': isNew,