feat: compelete bal pending screen

lib/data/services/api_client.dart

@@ -42,6 +42,33 @@ class ApiClient {
  * =================
 */
 
+  Future<Result<Bal>> startBal(int id) async {
+    final client = Client();
+    try {
+      final headers = await _getHeaders();
+      final response = await client.post(
+        Uri.parse("https://$apiBasePath/bal/${id.toString()}/start"),
+        headers: headers,
+      );
+      if (response.statusCode == 200) {
+        final json = jsonDecode(response.body);
+        return Result.ok(Bal.fromJSON(json));
+      } else if (response.statusCode == 403) {
+        throw "You don't own the specified BAL";
+      } else if (response.statusCode == 404) {
+        throw "No BAL with specified ID found";
+      } else if (response.statusCode == 409) {
+        throw "Cannot have multiple BAl ongoing at the same time!";
+      } else {
+        throw "Unknown error";
+      }
+    } catch (e) {
+      return Result.error(Exception(e));
+    } finally {
+      client.close();
+    }
+  }
+
   Future<Result<Bal>> editBal(
     int id,
     String name,