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>

.storybook/preview.tsx

@@ -3,6 +3,7 @@ import type { Preview } from '@storybook/react';
 import { ThemeProvider } from '@mui/material/styles';
 import CssBaseline from '@mui/material/CssBaseline';
 import { theme } from '../src/theme';
+import '../src/tailwind.css';
 import '../src/theme/generated/tokens.css';
 
 const preview: Preview = {