Fix: Remove relatedPosts field causing KeyStatic error

The relationship field was causing "n.call is not a function" error.
Removed relatedPosts from both keystatic.config.ts and content.config.ts.

Other optimizations remain:
- Site singleton for global settings
- Brand mark (globe logo)
- Entry layout grouping
- Preview URL configuration
- Published and featured checkbox fields

Co-Authored-By: Claude <noreply@anthropic.com>

src/content.config.ts

@@ -14,7 +14,6 @@ const post = defineCollection({
     layout: z.string().default('post'),
     image: z.string().optional(),
     excerpt: z.string().optional(),
-    relatedPosts: z.array(z.string()).optional(),
   }),
 });