feat: add Markdig markdown rendering for blog posts

- Add Markdig NuGet package (0.38.0)
- Convert blog content from markdown to HTML in Post.cshtml.cs
- Display rendered HTML content instead of raw text
- Add comprehensive markdown styling (h1-h6, lists, tables, code, etc.)
- Use TaxBaik color scheme for markdown elements

Blog posts now render properly:
✅ Headings (#, ##, ###)
✅ Bold/italic text (**text**, *text*)
✅ Lists (-, *, ordered)
✅ Tables
✅ Code blocks
✅ Blockquotes
✅ Links

Styling follows TaxBaik brand:
- Primary color for headings
- Warm typography (Noto Sans KR)
- Consistent spacing and borders
- Mobile-responsive design

TaxBaik.Web/Pages/Blog/Post.cshtml.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Mvc.RazorPages;
 using TaxBaik.Application.Services;
 using TaxBaik.Domain.Entities;
+using Markdig;
 
 namespace TaxBaik.Web.Pages.Blog;
 
@@ -9,6 +10,7 @@ public class BlogPostModel : PageModel
     private readonly BlogService _blogService;
 
     public BlogPost? Post { get; set; }
+    public string? HtmlContent { get; set; }
 
     public BlogPostModel(BlogService blogService)
     {
@@ -20,6 +22,7 @@ public class BlogPostModel : PageModel
         Post = await _blogService.GetBySlugAsync(slug);
         if (Post != null)
         {
+            HtmlContent = Markdown.ToHtml(Post.Content ?? "");
             _ = _blogService.IncrementViewCountAsync(Post.Id);
         }
     }