fix accessbility issues

[masteradhoc]

What?

Fix two accessibility issues in the Two-Factor settings UI.

Why?

• The "Primary Method" <select> had no programmatically associated label, failing WCAG 1.3.1.
• The <fieldset id="two-factor-options"> had no <legend>, leaving screen readers without a group description for the contained controls.

How?

• Wrapped the "Primary Method" <th> text in <label for="two-factor-primary-provider"> and added the matching id to the <select>.
• Added <legend class="screen-reader-text">Two-Factor Options</legend> as the first child of the fieldset. The screen-reader-text class keeps it visually hidden since the <h2> above already provides the visible heading.

Testing Instructions

1. Open a user profile page with Two-Factor active.
2. Run an automated a11y scan (e.g. Axe, WAVE) — "select has no label" and "fieldset has no legend" errors should be gone.
3. Navigate with a screen reader (VoiceOver/NVDA) — the primary method select should announce "Primary Method" and each checkbox within the fieldset should be preceded by "Two-Factor Options".

Changelog Entry

Fixed - "Primary Method" select and Two-Factor Options fieldset were missing accessible labels for screen readers.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: masteradhoc <masteradhoc@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.