Chore: upgrade theme.json to version 3

[sagarjadhav]

Overview

WordPress 6.6 introduced theme.json version 3. Our theme currently uses version 2 and should be updated to stay current.

What needs to change

Bump "version" from 2 to 3 in theme.json.

Why no other changes are needed

The only breaking change in v3 is how default font size presets are handled. In v3, WordPress merges its default font sizes with the theme's, causing conflicts in the block editor. Adding "defaultFontSizes": false suppresses the WP defaults and ensures only the theme's custom sizes (small, medium, large, x-large, xx-large) are used.

Acceptance Criteria

• theme.json version is set to 3
• Font sizes render correctly in the block editor
• No unexpected preset values appear in the typography controls