Submission Review Guidelines:

• I have followed the WooCommerce Contributing Guidelines and the WordPress Coding Standards.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• I have reviewed my code for security best practices.
• Following the above guidelines will result in quick merges and clear and detailed feedback when appropriate.

Changes proposed in this Pull Request:

Replace the sidebar-based category navigation in the email template selection modal with a horizontal tabs layout, using the new @wordpress/ui Tabs component. This aligns the template picker with the updated email marketing flow design.

How to test the changes in this Pull Request:

1. Open the email editor (WooCommerce > Email > create or edit an email).
2. The template selection modal should appear with horizontal tabs at the top (e.g. "All designs", category tabs).
3. Verify clicking tabs filters templates correctly. "All designs" should show every template.
4. Verify keyboard navigation works on tabs: Arrow Left/Right, Home, End keys should move focus between tabs.
5. If swapping templates (editing an existing email), verify the "Recent" tab is hidden.
6. Verify the "Start from scratch" button in the footer works.
7. Resize the browser window and verify the template grid adjusts responsively (1 column → 2 → 3).

Screenshots or screen recordings:

Before	After

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