Compare parsed /test response instead of stringified Dio dump

Previously BackendAPI.test() called response.toString().contains('OOTT_API_OK'),
which scans the full stringified Response (status line, headers, body, request
path). A reverse proxy or captive portal returning HTML that happens to echo the
token would falsely pass the connectivity check. Compare response.data — the
JSON-decoded String the backend's /api/test handler returns — against the
expected value with exact equality instead.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/lib/utils/oott_api.dart

@@ -153,7 +153,7 @@ class BackendAPI {
 
     try {
       Response response = await dio.get('/test');
-      return response.toString().contains('OOTT_API_OK')
+      return response.data == 'OOTT_API_OK'
           ? null
           : "URL successfully called but didn't return the expected value. Check your URL and make sure it points to your OOTT backend base URL.";
     } catch (e) {