Fix: Fix focus shift on opening of modal popup in templates

[Sourav61]

What?

Closes #69708

This PR ensures that when a modal is opened, the focus shifts to the first interactive element inside the modal, such as the close button or an input field.

Why?

Currently, the focus behavior in some modals is inconsistent. In some cases, the parent container is not focusable due to tabindex=-1, preventing proper keyboard navigation. This fix ensures that focus lands on the first interactive element when the modal opens, improving accessibility and user experience.

How?

1. Used querySelector to find the first interactive element (button, [href], input, select, textarea, [tabindex]:not([tabindex="-1"])).
2. Wrapped the focus shift inside a setTimeout to ensure DOM readiness.
3. Added cleanup to useEffect to prevent memory leaks.

Testing Instructions

1. Open the WordPress site editor.
2. Click on a button that triggers a modal (e.g., "Add Template -> Custom Template").
3. Verify that focus automatically moves to the first interactive element in the modal.
4. Navigate using the keyboard (Tab and Shift + Tab) to confirm expected behavior.

Screenshots or screencast

modal-focus-shift-issue.mp4

[github-actions]

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Co-authored-by: Sourav61 <[email protected]>
Co-authored-by: Mayank-Tripathi32 <[email protected]>
Co-authored-by: t-hamano <[email protected]>

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