fix: checkbox logic (#1537)

## Description

<!--- Describe the changes introduced by this pull request. -->
<!--- Explain what problem it solves or what feature/fix it adds. -->

## Related Issue

<!--- If this pull request is related to a specific issue, reference it
here using #issue_number. -->
<!--- For example, "Fixes #123" or "Addresses #456". -->

## Changes Made

<!--- Provide a summary of the changes made in this pull request. -->
<!--- Include any relevant technical details or architecture changes.
-->

- Change 1
- Change 2
- ...

## Testing Performed

<!--- Describe the testing that you have performed to validate these
changes. -->
<!--- Include information about test cases, testing environments, and
results. -->

- Tested feature X in scenario Y.
- Ran unit tests for component Z.
- Tested on browsers A, B, and C.
- ...

## Checklist

<!--- Please check the boxes that apply to this pull request. -->
<!--- You can add or remove items as needed. -->

- [ ] I have tested these changes locally and they work as expected.
- [ ] I have added/updated tests that prove the effectiveness of these
changes.
- [ ] I have updated the documentation to reflect these changes, if
applicable.
- [ ] I have followed the project's coding style guidelines.
- [ ] I have addressed the code review feedback from the previous
submission, if applicable.

## Additional Notes

<!--- Provide any additional context or notes for the reviewers. -->
<!--- This might include details about design decisions, potential
concerns, or anything else relevant. -->

packages/lib/universal/field-checkbox.ts

@@ -0,0 +1,21 @@
+export const fromCheckboxValue = (customText: string): string[] => {
+  if (!customText) {
+    return [];
+  }
+
+  try {
+    const parsed = JSON.parse(customText);
+
+    if (!Array.isArray(parsed)) {
+      throw new Error('Parsed checkbox values are not an array');
+    }
+
+    return parsed;
+  } catch {
+    return customText.split(',').filter(Boolean);
+  }
+};
+
+export const toCheckboxValue = (values: string[]): string => {
+  return JSON.stringify(values);
+};