add changeset comment tracking and message notification bubble on startup

lib/widgets/unread_notifications_dialog.dart

@@ -0,0 +1,54 @@
+import 'package:flutter/material.dart';
+import '../services/localization_service.dart';
+
+/// Dialog shown once per session when the user has unread OSM messages
+/// and/or unread changeset comments, prompting them to view their OSM
+/// account details.
+class UnreadNotificationsDialog extends StatelessWidget {
+  final VoidCallback onView;
+  final VoidCallback onDismiss;
+
+  const UnreadNotificationsDialog({
+    super.key,
+    required this.onView,
+    required this.onDismiss,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    final locService = LocalizationService.instance;
+
+    return AlertDialog(
+      title: Row(
+        children: [
+          Icon(
+            Icons.notifications_active_outlined,
+            color: Theme.of(context).colorScheme.primary,
+          ),
+          const SizedBox(width: 8),
+          Expanded(
+            child: Text(locService.t('auth.unreadNotificationsTitle')),
+          ),
+        ],
+      ),
+      content: Text(locService.t('auth.unreadNotificationsMessage')),
+      actions: [
+        TextButton(
+          onPressed: () {
+            Navigator.of(context).pop();
+            onDismiss();
+          },
+          child: Text(locService.t('auth.reauthLater')),
+        ),
+        FilledButton.icon(
+          onPressed: () {
+            Navigator.of(context).pop();
+            onView();
+          },
+          icon: const Icon(Icons.arrow_forward, size: 18),
+          label: Text(locService.t('auth.unreadNotificationsView')),
+        ),
+      ],
+    );
+  }
+}