Add Tailwind v3 integration aligned with production config

- Create tailwind.config.js merging production tokens with design
  system palettes from Style Dictionary
- Extend Style Dictionary to output tailwind-tokens.js module
- Add PostCSS config and Tailwind directive CSS
- Import Tailwind CSS in Storybook preview
- Production breakpoints, colours, typography, spacing preserved
- Design system brand/sage/neutral/red/amber/green/blue palettes
  added alongside production's generic color-1..6 naming

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+};