chore: scaffold Astro 5 project with token/style layer

- Astro 5 static config (trailing-slash canonical, i18n en-root/no-prefixed)
- Tailwind 4 over CSS custom-property tokens (both themes via [data-theme])
- design tokens: colour/spacing/type/radius/motion per Phase 1 specs
- motion layer with reduced-motion overrides
- strict TypeScript, ESLint flat config, Prettier
- self-hosted variable fonts (Space Grotesk / Inter / JetBrains Mono)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

site/.prettierrc.json

@@ -0,0 +1,8 @@
+{
+  "printWidth": 100,
+  "singleQuote": true,
+  "semi": true,
+  "trailingComma": "all",
+  "plugins": ["prettier-plugin-astro", "prettier-plugin-tailwindcss"],
+  "overrides": [{ "files": "*.astro", "options": { "parser": "astro" } }]
+}