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>

keystatic.config.ts

@@ -1,58 +1,19 @@
-import { config, fields, collection, singleton } from '@keystatic/core';
-
-// Use GitHub storage in production (when env vars are set), local storage for development
-const isProd = import.meta.env.PUBLIC_KEYSTATIC_GITHUB_APP_SLUG;
+import { config, fields, collection } from '@keystatic/core';
 
 export default config({
-  storage: isProd ? {
+  storage: {
     kind: 'github',
     repo: 'YM1KTC/ARC-Web-Sitesi',
     branch: 'main',
-  } : undefined,
+  },
   ui: {
     brand: {
       name: 'ARC CMS',
-      mark: () => '<svg width="24" height="24" viewBox="0 0 24 24" fill="currentColor"><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm-1 17.93c-3.95-.49-7-3.85-7-7.93 0-.62.08-1.21.21-1.79L9 15v1c0 1.1.9 2 2 2v1.93zm6.9-2.54c-.26-.81-1-1.39-1.9-1.39h-1v-3c0-.55-.45-1-1-1H8v-2h2c.55 0 1-.45 1-1V7h2c1.1 0 2-.9 2-2v-.41c2.93 1.19 5 4.06 5 7.41 0 2.08-.8 3.97-2.1 5.39z"/></svg>',
     },
     navigation: {
       'İçerik': ['post'],
-      'Ayarlar': ['site'],
     },
   },
-  singletons: {
-    site: singleton({
-      label: 'Site Ayarları',
-      path: 'src/data/site',
-      schema: {
-        siteName: fields.text({
-          label: 'Site Adı',
-          default: 'Amatör Radyocular Derneği',
-        }),
-        siteDescription: fields.text({
-          label: 'Site Açıklaması',
-          multiline: true,
-          default: 'Amatör telsizciliği yaygınlaştıran Türkiye\'deki öncü dernek.',
-        }),
-        siteUrl: fields.text({
-          label: 'Site URL',
-          default: 'https://radio.org.tr',
-        }),
-        contactEmail: fields.text({
-          label: 'İletişim E-posta',
-          default: 'info@radio.org.tr',
-        }),
-        socialMedia: fields.object({
-          label: 'Sosyal Medya Hesapları',
-          schema: {
-            facebook: fields.text({ label: 'Facebook' }),
-            twitter: fields.text({ label: 'Twitter/X' }),
-            instagram: fields.text({ label: 'Instagram' }),
-            youtube: fields.text({ label: 'YouTube' }),
-          },
-        }),
-      },
-    }),
-  },
   collections: {
     post: collection({
       label: 'Blog Yazıları',
@@ -62,15 +23,7 @@ export default config({
         contentField: 'body',
         frontmatter: 'yaml',
       },
-      previewURL: ({ slug }) => `http://localhost:4321/blog/${slug}`,
-      columns: ['title', 'date', 'author', 'published', 'featured'],
-      entryLayout: {
-        'Ana Bilgiler': ['title', 'date', 'author', 'published', 'featured'],
-        'Medya': ['image'],
-        'Sınıflandırma': ['categories', 'tags', 'layout'],
-        'Özet': ['excerpt'],
-        'İçerik': ['body'],
-      },
+      columns: ['title', 'date', 'author'],
       schema: {
         title: fields.slug({
           name: {
@@ -87,16 +40,6 @@ export default config({
           description: 'Çağrı işareti (örn: TA1SPH) veya yazar adı',
           default: 'TA1SPH',
         }),
-        published: fields.checkbox({
-          label: 'Yayınlandı',
-          description: 'Bu yazı yayınlandı mı?',
-          defaultValue: true,
-        }),
-        featured: fields.checkbox({
-          label: 'Öne Çıkan',
-          description: 'Ana sayfada öne çıkan yazılar',
-          defaultValue: false,
-        }),
         categories: fields.array(
           fields.text({
             label: 'Kategori',