asw-v01: drop OpenProps, native CSS tokens system

- Replace OpenProps (~25KB) with native src/tokens.css (~40 tokens)
- Remove open-props, postcss-import from package.json + postcss.config
- Update main.css: remove @import open-props/*, import ./tokens.css
- Rewrite 01-tokens.css header to reference tokens.css
- All raw OpenProps values (font stacks, sizes, weights, colors,
  easings, durations, shadows, radii, animations, media queries)
  now defined natively in tokens.css
- Build uses cat concat + cssnano (no postcss-import needed)
- Build output: 78KB minified (saved ~24KB from OpenProps removal)

postcss.config.js

@@ -1,6 +1,5 @@
 module.exports = {
   plugins: [
-    require('postcss-import'),
     require('postcss-custom-media'),
     require('cssnano')({ preset: 'default' })
   ]