Add dio_smart_retry for automatic GET request retries

Transient network hiccups and temporary backend errors (5xx, timeouts)
now retry transparently up to twice (1 s then 3 s backoff) before
surfacing an error to the user. Non-idempotent methods (POST, PUT,
DELETE), cancellations, and non-retryable status codes are excluded.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/utils/oott_api.dart

@@ -1,6 +1,7 @@
 import 'dart:io';
 
 import 'package:dio/dio.dart';
+import 'package:dio_smart_retry/dio_smart_retry.dart';
 import 'package:encrypter/encrypter/xor.dart';
 import 'package:flutter/foundation.dart';
 import 'package:frontend/utils/pref_utils.dart';
@@ -16,6 +17,25 @@ const _connectTimeout = Duration(seconds: 5);
 const _receiveTimeout = Duration(seconds: 15);
 const _sendTimeout = Duration(seconds: 15);
 
+const _retryableStatuses = {408, 429, 500, 502, 503, 504};
+
+RetryInterceptor _buildRetryInterceptor(Dio dio) => RetryInterceptor(
+  dio: dio,
+  retries: 2,
+  retryDelays: const [Duration(seconds: 1), Duration(seconds: 3)],
+  logPrint: kDebugMode ? (msg) => debugPrint(msg.toString()) : null,
+  retryEvaluator: (error, attempt) {
+    if (error.requestOptions.method != 'GET') return false;
+    if (error.type == DioExceptionType.cancel) return false;
+    if (error.error is FormatException) return false;
+    if (error.type == DioExceptionType.badResponse) {
+      final status = error.response?.statusCode;
+      return status != null && _retryableStatuses.contains(status);
+    }
+    return true;
+  },
+);
+
 LogInterceptor _buildLogInterceptor() => LogInterceptor(
   request: true,
   requestHeader: false,
@@ -87,6 +107,7 @@ class BackendAPI {
         },
       ),
     );
+    _dio.interceptors.add(_buildRetryInterceptor(_dio));
     if (kDebugMode) {
       _dio.interceptors.add(_buildLogInterceptor());
     }

frontend/pubspec.lock

@@ -89,6 +89,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "5.9.1"
+  dio_smart_retry:
+    dependency: "direct main"
+    description:
+      name: dio_smart_retry
+      sha256: c8e20da5f49289fa7dce5c9c6b5b120928e3661aefa0fa2d206ea6d93f580928
+      url: "https://pub.dev"
+    source: hosted
+    version: "7.0.1"
   dio_web_adapter:
     dependency: transitive
     description:

frontend/pubspec.yaml

@@ -13,6 +13,7 @@ dependencies:
   intl: ^0.20.2
   provider: ^6.1.5
   dio: ^5.9.1
+  dio_smart_retry: ^7.0.1
   encrypter: ^2.0.0
   shared_preferences: ^2.5.4
   fl_chart: ^1.2.0