Add back columns for better post list display

Shows title, date, and author columns in the post list view.

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

keystatic.config.ts

@@ -23,6 +23,7 @@ export default config({
         contentField: 'body',
         frontmatter: 'yaml',
       },
+      columns: ['title', 'date', 'author'],
       schema: {
         title: fields.text({
           label: 'Başlık',