feat: add custom slug validation and handling for posts

- Enhance content configuration with slug validation to ensure only valid characters are used
- Update post routing to support custom slugs with fallback to default slug
- Implement slug duplication check to prevent conflicts across different languages
- Modify various page components to use custom or default slugs in URL generation

src/pages/rss.xml.ts

@@ -42,7 +42,7 @@ export async function GET() {
       pubDate: post.data.published,
       description: post.data.description || getExcerpt(post.body),
       // Generate absolute URL for post
-      link: new URL(`posts/${post.slug}/`, url).toString(),
+      link: new URL(`posts/${post.data.slug || post.slug}/`, url).toString(),
       // Convert markdown content to HTML, allowing img tags
       content: post.body
         ? sanitizeHtml(parser.render(post.body), {