Add KeyStatic CMS optimizations

- Add site singleton for global settings (name, description, URL, contact, social media)
- Add brand mark (globe icon) to KeyStatic header
- Add entry layout grouping (Ana Bilgiler, Medya, Sınıflandırma, Özet, İçerik)
- Add preview URL configuration for local development
- Add published, featured, and relatedPosts fields
- Use local storage for development, GitHub storage for production
- Update Astro content schema with new fields

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

src/content.config.ts

@@ -7,11 +7,14 @@ 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'),
     image: z.string().optional(),
     excerpt: z.string().optional(),
+    relatedPosts: z.array(z.string()).optional(),
   }),
 });