add stats page

lib/services/data_service/data_service_stats.dart

@@ -0,0 +1,28 @@
+part of 'data_service.dart';
+
+extension DataServiceStats on DataService {
+  Future<void> fetchAboutStats({bool force = false}) async {
+    if (_isAboutStatsLoading) return;
+    if (!force && _aboutStats != null) return;
+    _isAboutStatsLoading = true;
+    _notifyAsync();
+    try {
+      final json = await api.get('/stats/about');
+      if (json is Map<String, dynamic>) {
+        _aboutStats = StatsAbout.fromJson(json);
+      } else if (json is Map) {
+        _aboutStats = StatsAbout.fromJson(
+          json.map((key, value) => MapEntry(key.toString(), value)),
+        );
+      } else {
+        throw Exception('Unexpected stats response: $json');
+      }
+    } catch (e) {
+      debugPrint('Failed to fetch about stats: $e');
+      _aboutStats = null;
+    } finally {
+      _isAboutStatsLoading = false;
+      _notifyAsync();
+    }
+  }
+}