Build theme tokens reference along with patterns

It might be nice to build a reference page for the theme tokens defined in theme.json, in addition to just screenshotting patterns. The same process (playwright script to take screenshots of content output by an endpoint served by the plugin) could be easily used to generate this reference page.

Ideas for content that would be good to show might be:
- color palette
- typography scale
- font families and sizes
- spacing
- style variants of core blocks like buttons, etc.

Start by locating theme.json, the existing Playwright screenshot script, and the plugin endpoint that serves pattern content. Review how pattern screenshots are generated, then define a reference page showing the theme tokens and core-block style variants; done means the page is generated through the same process and includes the agreed token categories.

Written by the indexing model from the issue text.