feat: add remote products and qr code for payconic

lib/services/kooltap_api_client.dart

@@ -104,6 +104,43 @@ class KoolTapApiClient {
     );
   }
 
+  /// Uploads a picture taken on this tablet; the server crops and stores it
+  /// and returns its id, which a `product_updated` event then links.
+  Future<String> uploadImage({
+    required String token,
+    required String deviceId,
+    required List<int> bytes,
+    required String filename,
+  }) async {
+    final request =
+        http.MultipartRequest(
+            'POST',
+            Uri.parse('$baseUrl/devices/$deviceId/images'),
+          )
+          ..headers['Authorization'] = 'Bearer $token'
+          ..files.add(
+            http.MultipartFile.fromBytes('file', bytes, filename: filename),
+          );
+    // Uploading a photo over a slow connection takes longer than a sync
+    final response = await http.Response.fromStream(
+      await _client.send(request).timeout(_timeout * 4),
+    );
+
+    final data = _decode(response, authenticated: true) as Map<String, dynamic>;
+    return data['imageId'] as String;
+  }
+
+  /// A product picture (public: pictures never change at their address).
+  Future<List<int>> downloadImage(String imageId) async {
+    final response = await _client
+        .get(Uri.parse('$baseUrl/images/$imageId'))
+        .timeout(_timeout * 2);
+    if (response.statusCode != 200) {
+      throw ApiException(response.statusCode, 'Image $imageId not available');
+    }
+    return response.bodyBytes;
+  }
+
   Future<Object?> _post(
     String path,
     Map<String, dynamic> body, {