Fix blog category selection and empty state rendering

src/TaxBaik.Application/DTOs/CreateBlogPostDto.cs

@@ -4,7 +4,7 @@ public class CreateBlogPostDto
 {
     public required string Title { get; set; }
     public required string Content { get; set; }
-    public int? CategoryId { get; set; }
+    public int CategoryId { get; set; }
     public string? Tags { get; set; }
     public string? SeoTitle { get; set; }
     public string? SeoDescription { get; set; }
@@ -18,7 +18,7 @@ public class BlogPostResponseDto
     public int Id { get; set; }
     public string Title { get; set; } = string.Empty;
     public string Content { get; set; } = string.Empty;
-    public int? CategoryId { get; set; }
+    public int CategoryId { get; set; }
     public string? Tags { get; set; }
     public string? SeoTitle { get; set; }
     public string? SeoDescription { get; set; }

src/TaxBaik.Application/DTOs/Validators.cs

@@ -8,6 +8,7 @@ public sealed class CreateBlogPostDtoValidator : AbstractValidator<CreateBlogPos
     {
         RuleFor(x => x.Title).NotEmpty().MaximumLength(ValidationRules.TitleMaxLength);
         RuleFor(x => x.Content).NotEmpty().MinimumLength(ValidationRules.MessageMinLength).MaximumLength(ValidationRules.ContentMaxLength);
+        RuleFor(x => x.CategoryId).GreaterThan(0);
         RuleFor(x => x.Tags).MaximumLength(ValidationRules.TagsMaxLength);
         RuleFor(x => x.SeoTitle).MaximumLength(ValidationRules.SeoTitleMaxLength);
         RuleFor(x => x.SeoDescription).MaximumLength(ValidationRules.SeoDescriptionMaxLength);