feat: more easter eggs + version checking

lib/services/kooltap_api_client.dart

@@ -26,6 +26,13 @@ class UnauthorizedException extends ApiException {
     : super(401, message ?? 'Unauthorized');
 }
 
+/// This app is older than the backoffice accepts; it has to be updated
+/// before it can sync again. The token stays valid.
+class UpdateRequiredException extends ApiException {
+  const UpdateRequiredException({super.code, String? message})
+    : super(426, message ?? 'App update required');
+}
+
 class RateLimitedException extends ApiException {
   final Duration retryAfter;
 
@@ -43,15 +50,26 @@ class KoolTapApiClient {
   final http.Client _client;
   final String baseUrl;
 
-  KoolTapApiClient({http.Client? client, this.baseUrl = kRemoteServerUrl})
-    : _client = client ?? http.Client();
+  /// Sent with every request so the backoffice can refuse an app that is too
+  /// old (`426 app_update_required`).
+  final String? appVersion;
+
+  KoolTapApiClient({
+    http.Client? client,
+    this.baseUrl = kRemoteServerUrl,
+    this.appVersion,
+  }) : _client = client ?? http.Client();
+
+  Map<String, String> get _versionHeader => {
+    'X-App-Version': ?appVersion,
+  };
 
   static const _timeout = Duration(seconds: 15);
   static const _defaultRetryAfter = Duration(seconds: 60);
 
   Future<List<Venue>> getVenues() async {
     final response = await _client
-        .get(Uri.parse('$baseUrl/venues/'))
+        .get(Uri.parse('$baseUrl/venues/'), headers: _versionHeader)
         .timeout(_timeout);
 
     final data = _decode(response) as List<dynamic>;
@@ -117,6 +135,7 @@ class KoolTapApiClient {
             'POST',
             Uri.parse('$baseUrl/devices/$deviceId/images'),
           )
+          ..headers.addAll(_versionHeader)
           ..headers['Authorization'] = 'Bearer $token'
           ..files.add(
             http.MultipartFile.fromBytes('file', bytes, filename: filename),
@@ -133,7 +152,7 @@ class KoolTapApiClient {
   /// A product picture (public: pictures never change at their address).
   Future<List<int>> downloadImage(String imageId) async {
     final response = await _client
-        .get(Uri.parse('$baseUrl/images/$imageId'))
+        .get(Uri.parse('$baseUrl/images/$imageId'), headers: _versionHeader)
         .timeout(_timeout * 2);
     if (response.statusCode != 200) {
       throw ApiException(response.statusCode, 'Image $imageId not available');
@@ -151,6 +170,7 @@ class KoolTapApiClient {
           Uri.parse('$baseUrl$path'),
           headers: {
             'Content-Type': 'application/json',
+            ..._versionHeader,
             if (token != null) 'Authorization': 'Bearer $token',
           },
           body: jsonEncode(body),
@@ -170,6 +190,11 @@ class KoolTapApiClient {
       throw UnauthorizedException(code: error.code, message: error.message);
     }
 
+    if (status == 426) {
+      final error = _error(response);
+      throw UpdateRequiredException(code: error.code, message: error.message);
+    }
+
     if (status == 429) {
       final seconds = int.tryParse(response.headers['retry-after'] ?? '');
       throw RateLimitedException(

lib/services/remote_sync_service.dart

@@ -273,6 +273,13 @@ class RemoteSyncService {
       stopPeriodicSync();
       _emit(RemoteSyncEventType.unauthorized);
       return;
+    } on UpdateRequiredException catch (e) {
+      // Retrying can't help until the app is updated. The token and every
+      // outbox stay, so nothing is lost; a manual sync or restart tries again
+      debugPrint('RemoteSyncService: app update required');
+      stopPeriodicSync();
+      _emit(RemoteSyncEventType.updateRequired, message: e.message);
+      return;
     } on RateLimitedException catch (e) {
       nextDelay = await _recordFailure(e, retryAfter: e.retryAfter);
     } catch (e, stack) {
@@ -328,6 +335,8 @@ class RemoteSyncService {
         );
       } on UnauthorizedException {
         rethrow;
+      } on UpdateRequiredException {
+        rethrow;
       } catch (e) {
         debugPrint('RemoteSyncService: picture upload failed: $e');
       }