feat: add sitemap integration and configure site URL

- Add @astrojs/sitemap dependency and configure it in astro.config.mjs
- Set site URL and markdown theme in astro config
- Remove unused theme variables from global.css

package.json

@@ -17,6 +17,7 @@
   },
   "dependencies": {
     "@astrojs/react": "^4.2.1",
+    "@astrojs/sitemap": "^3.4.1",
     "@radix-ui/react-scroll-area": "^1.2.9",
     "@radix-ui/react-slot": "^1.1.2",
     "@tailwindcss/vite": "^4.0.14",