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/TaxBaik.Web.csproj

@@ -23,6 +23,7 @@
     <PackageReference Include="Serilog.AspNetCore" Version="8.0.1" />
     <PackageReference Include="Serilog.Sinks.Console" Version="6.0.0" />
     <PackageReference Include="Serilog.Sinks.File" Version="5.0.0" />
+    <PackageReference Include="Markdig" Version="0.38.0" />
   </ItemGroup>
 
 </Project>