fix: use local diagram tokens to avoid circular build dependency

- Create _diagram-tokens.scss with static token values from source
- Update Mermaid.scss to import local tokens instead of generated dist/
- Fixes CI build failure where diagram tokens weren't available
- ProductCard.scss formatting cleanup

The diagram tokens are now defined locally in the source, eliminating
the dependency on generated files in the gitignored dist/ folder.
This ensures the component CSS can build even before tokens are generated.

Author: Fergus Bisset