Use heading instead of paragraph for menu title in overlay contents pattern - #109

Description

Fixes #101

In the Overlay Contents template part pattern (patterns/overlay-contents.php), the "Menu" title preceding the navigation block was rendered using a paragraph (<p> tag with wp:paragraph block markup).

As identified in #101, because this text introduces and describes the navigation structure directly following it, semantics and document outline guidelines require a heading (<h2>) rather than a paragraph.

Changes:

• Replaced wp:paragraph with wp:heading in patterns/overlay-contents.php.
• Rendered <h2 class="wp-block-heading ..."> instead of <p ...>, retaining all existing styling tokens, colors, typography, font size, and translation context.
• Updated metadata text domain context string from 'Name of the navigation title paragraph' to 'Name of the navigation title heading'.

Testing Instructions:

1. In the Site Editor, open a template that contains the Navigation block (or open Patterns > Overlay Contents).
2. Inspect the "Menu" title above the navigation links.
3. Confirm that it is rendered as an <h2> heading with class wp-block-heading, preserving identical visual styling.
4. Verify PHP syntax across the theme with php -l patterns/overlay-contents.php.