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>

astro.config.mjs

@@ -1,6 +1,6 @@
 import { defineConfig } from 'astro/config';
-import tailwind from '@astrojs/tailwind';
+import UnoCSS from '@unocss/astro';
 
 export default defineConfig({
-  integrations: [tailwind()],
+  integrations: [UnoCSS({ injectReset: true })],
 });