feat: replace Tailwind with UnoCSS

Faster dev HMR, native Astro 6 support, same utility class names.
Tokens migrated to uno.config.ts.

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

src/styles/global.css

@@ -1,3 +1 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+/* UnoCSS handles reset and utilities via astro.config.mjs injectReset */