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_test.go

@@ -0,0 +1,81 @@
+package markdown
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestCodeBlockWithTitle(t *testing.T) {
+	input := "```typescript title=\"app.ts\"\nconst x = 1\n```"
+
+	result, err := Render(input)
+	if err != nil {
+		t.Fatalf("Render failed: %v", err)
+	}
+
+	// Check that the code-block wrapper is present
+	if !strings.Contains(result, `class="code-block"`) {
+		t.Error("Expected code-block wrapper class")
+	}
+
+	// Check that the header with title is present
+	if !strings.Contains(result, `class="code-title"`) {
+		t.Error("Expected code-title class")
+	}
+
+	// Check that the title text is rendered
+	if !strings.Contains(result, "app.ts") {
+		t.Error("Expected title 'app.ts' in output")
+	}
+
+	// Check that the icon is present
+	if !strings.Contains(result, `class="code-icon"`) {
+		t.Error("Expected code-icon class")
+	}
+
+	// Check that the copy button is present
+	if !strings.Contains(result, `class="code-copy"`) {
+		t.Error("Expected code-copy class")
+	}
+
+	t.Log("Rendered output:")
+	t.Log(result)
+}
+
+func TestCodeBlockWithoutTitle(t *testing.T) {
+	input := "```go\nfmt.Println(\"hello\")\n```"
+
+	result, err := Render(input)
+	if err != nil {
+		t.Fatalf("Render failed: %v", err)
+	}
+
+	// Should still have header with language
+	if !strings.Contains(result, `class="code-block"`) {
+		t.Error("Expected code-block wrapper for language-only block")
+	}
+
+	if !strings.Contains(result, "go") {
+		t.Error("Expected language 'go' in output")
+	}
+
+	t.Log("Rendered output:")
+	t.Log(result)
+}
+
+func TestCodeBlockPlain(t *testing.T) {
+	input := "```\nplain code\n```"
+
+	result, err := Render(input)
+	if err != nil {
+		t.Fatalf("Render failed: %v", err)
+	}
+
+	// Plain code blocks without language/title should not have header
+	if strings.Contains(result, `class="code-header"`) {
+		t.Error("Plain code blocks should not have header")
+	}
+
+	t.Log("Rendered output:")
+	t.Log(result)
+}