Add Font Weight option to Button element

[jjroelofs]

Proposal

Add a "Font Weight" option to the theme settings for buttons, allowing site builders to choose between Normal (default) and Bold font weight for all buttons.

Currently there are no button-specific design controls in the theme settings. This would be the first button styling option, following the same pattern used for heading font weight (headings_bold).

The implementation should:

• Add a checkbox form field for bold button text
• Register the CSS variable via the existing CSS variables system
• Apply the font-weight CSS property to .btn selectors