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/themes.go

@@ -4,6 +4,7 @@ import (
 	"strings"
 
 	"github.com/alecthomas/chroma/v2"
+	chromahtml "github.com/alecthomas/chroma/v2/formatters/html"
 	"github.com/alecthomas/chroma/v2/styles"
 )
 
@@ -150,3 +151,18 @@ func GenerateHljsCSS(themeName string) (string, error) {
 func ListThemes() []string {
 	return styles.Names()
 }
+
+func GenerateChromaCSS(themeName string) (string, error) {
+	style := styles.Get(themeName)
+	if style == nil {
+		style = styles.Get("github")
+	}
+
+	formatter := chromahtml.New(chromahtml.WithClasses(true))
+	var css strings.Builder
+	css.WriteString("/* Chroma theme: " + themeName + " */\n")
+	if err := formatter.WriteCSS(&css, style); err != nil {
+		return "", err
+	}
+	return css.String(), nil
+}