add stats page

lib/services/api_service.dart

@@ -128,7 +128,12 @@ class ApiService {
       await _onUnauthorized!();
     }
 
-    throw Exception('API error ${res.statusCode}: $body');
+    final message = _extractErrorMessage(body);
+    throw ApiException(
+      statusCode: res.statusCode,
+      message: message,
+      body: body,
+    );
   }
 
   dynamic _decodeBody(http.Response res) {
@@ -149,4 +154,41 @@ class ApiService {
       return res.body;
     }
   }
+
+  String _extractErrorMessage(dynamic body) {
+    if (body == null) return 'No response body';
+    if (body is String) return body;
+    if (body is Map<String, dynamic>) {
+      for (final key in ['message', 'error', 'detail', 'msg']) {
+        final val = body[key];
+        if (val is String && val.trim().isNotEmpty) return val;
+      }
+      return body.toString();
+    }
+    if (body is List) {
+      final parts = body
+          .map((e) => e is Map
+              ? _extractErrorMessage(Map<String, dynamic>.from(e))
+              : e.toString())
+          .where((e) => e.trim().isNotEmpty)
+          .toList();
+      if (parts.isNotEmpty) return parts.join('; ');
+    }
+    return body.toString();
+  }
+}
+
+class ApiException implements Exception {
+  final int statusCode;
+  final String message;
+  final dynamic body;
+
+  ApiException({
+    required this.statusCode,
+    required this.message,
+    this.body,
+  });
+
+  @override
+  String toString() => 'API error $statusCode: $message';
 }