feat: more easter eggs + version checking

test/kooltap_api_client_test.dart

@@ -88,5 +88,44 @@ void main() {
         throwsA(isA<UnauthorizedException>()),
       );
     });
+
+    test('sends the app version', () async {
+      late http.Request sent;
+      final client = KoolTapApiClient(
+        client: MockClient((request) async {
+          sent = request;
+          return http.Response('{}', 200);
+        }),
+        baseUrl: 'https://api.test',
+        appVersion: '1.4.0',
+      );
+
+      await client.sync(
+        token: 'token',
+        request: const SyncRequest(deviceId: 'device', openTabs: []),
+      );
+
+      expect(sent.headers['X-App-Version'], '1.4.0');
+    });
+
+    test('treats a 426 as an app that must be updated', () async {
+      final client = _clientReturning(
+        _error(426, 'app_update_required', 'Update the app to version 1.4.0'),
+      );
+
+      await expectLater(
+        client.sync(
+          token: 'token',
+          request: const SyncRequest(deviceId: 'device', openTabs: []),
+        ),
+        throwsA(
+          isA<UpdateRequiredException>().having(
+            (e) => e.message,
+            'message',
+            'Update the app to version 1.4.0',
+          ),
+        ),
+      );
+    });
   });
 }

test/remote_sync_service_test.dart

@@ -243,6 +243,22 @@ void main() {
       expect(await service.isConfigured, false);
     });
 
+    test('stops syncing but keeps the token when the app is too old', () async {
+      server.syncResponse = http.Response(
+        jsonEncode({
+          'error': {'code': 'app_update_required', 'message': 'Update'},
+        }),
+        426,
+      );
+
+      await service.sync();
+      await pumpEventQueue();
+
+      expect(eventTypes(), [RemoteSyncEventType.updateRequired]);
+      expect(events.single.message, 'Update');
+      expect(await service.isConfigured, true);
+    });
+
     test('creates planned tabs that are not open yet, once', () async {
       await barTabService.createTab(customerName: 'Alice');
       server.respondWith({