Revert KeyStatic to working state (Turkish UI only)

Removed all experimental features that were causing errors:
- Site singleton
- Brand mark (logo)
- Entry layout grouping
- Preview URL
- Published/featured checkbox fields
- Related posts field

Kept only the working Turkish UI customization:
- Brand name: "ARC CMS"
- Navigation: "İçerik" for posts

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

src/content.config.ts

@@ -7,8 +7,6 @@ const post = defineCollection({
     title: z.string(),
     date: z.date(),
     author: z.string().default('TA1SPH'),
-    published: z.boolean().default(true),
-    featured: z.boolean().default(false),
     categories: z.array(z.string()).default([]),
     tags: z.array(z.string()).default([]),
     layout: z.string().default('post'),