feat(core): switch to native fetch API for all HTTP requests

ts/bunq.classes.account.ts

@@ -161,7 +161,7 @@ export class BunqAccount {
     }
 
     // Sandbox user creation doesn't require authentication
-    const response = await plugins.smartrequest.request(
+    const response = await fetch(
       'https://public-api.sandbox.bunq.com/v1/sandbox-user-person',
       {
         method: 'POST',
@@ -170,12 +170,18 @@ export class BunqAccount {
           'User-Agent': 'bunq-api-client/1.0.0',
           'Cache-Control': 'no-cache'
         },
-        requestBody: '{}'
+        body: '{}'
       }
     );
 
-    if (response.body.Response && response.body.Response[0] && response.body.Response[0].ApiKey) {
-      return response.body.Response[0].ApiKey.api_key;
+    if (!response.ok) {
+      throw new Error(`Failed to create sandbox user: HTTP ${response.status}`);
+    }
+
+    const responseData = await response.json();
+
+    if (responseData.Response && responseData.Response[0] && responseData.Response[0].ApiKey) {
+      return responseData.Response[0].ApiKey.api_key;
     }
 
     throw new Error('Failed to create sandbox user');