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

src/styles/global.css

@@ -2,11 +2,6 @@
 @import "tw-animate-css";
 @plugin "@tailwindcss/typography";
 
-@theme {
-  --typography-code-before-content: "";
-  --typography-code-after-content: "";
-}
-
 /* Blog List Component Styles */
 .line-clamp-3 {
   display: -webkit-box;