feat: enhance accessibility and language support across components

- Updated various components to include `lang` attributes for improved accessibility and SEO.
- Introduced a new `env.d.ts` file to define environment variables for better type safety.
- Adjusted the print width in `.prettierrc.mjs` for improved code formatting.
- Streamlined the `Header`, `PostElement`, and `SocialLinks` components for better structure and clarity.
- Added language support to blog posts and updated the layout to reflect these changes.

src/pages/404.astro

@@ -3,9 +3,10 @@ import Layout from "../layouts/BaseLayout.astro";
 
 const title = "404 — Page Not Found | Valentin Popov";
 const description = "The page you're looking for doesn't exist!";
+const lang = "en";
 ---
 
-<Layout title={title} description={description}>
+<Layout title={title} description={description} lang={lang}>
 	<div style="text-align:center;">
 		<h1>404</h1>
 		<p><strong>Page not found</strong></p>