feat(markdown): add enhanced code blocks with syntax highlighting

- Add codeblock.go for custom Goldmark renderer
- Add code block header with language icon, filename, copy button
- Use Chroma for syntax highlighting with class-based output
- Add GenerateChromaCSS for theme CSS generation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/markdown/markdown.go

@@ -6,7 +6,6 @@ import (
 
 	"github.com/yuin/goldmark"
 	emoji "github.com/yuin/goldmark-emoji"
-	highlighting "github.com/yuin/goldmark-highlighting/v2"
 	"github.com/yuin/goldmark/extension"
 	"github.com/yuin/goldmark/parser"
 	"github.com/yuin/goldmark/renderer/html"
@@ -41,9 +40,7 @@ func getRenderer(codeTheme string) goldmark.Markdown {
 			extension.GFM,
 			extension.Typographer,
 			emoji.Emoji,
-			highlighting.NewHighlighting(
-				highlighting.WithStyle(codeTheme),
-			),
+			NewCodeBlockExtension(codeTheme),
 		),
 		goldmark.WithParserOptions(
 			parser.WithAutoHeadingID(),