feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

backend/pkg/logger/logger.go

@@ -0,0 +1,84 @@
+package logger
+
+import (
+	"os"
+
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	log *logrus.Logger
+)
+
+func Init() {
+	log = logrus.New()
+	log.SetOutput(os.Stdout)
+	log.SetFormatter(&logrus.JSONFormatter{
+		TimestampFormat: "2006-01-02 15:04:05",
+	})
+	log.SetLevel(logrus.InfoLevel)
+}
+
+func Info(msg string, fields ...interface{}) {
+	if len(fields) > 0 {
+		log.WithFields(logrus.Fields{"message": msg}).Info()
+	} else {
+		log.Info(msg)
+	}
+}
+
+func Infof(format string, args ...interface{}) {
+	log.Infof(format, args...)
+}
+
+func Debug(msg string, fields ...interface{}) {
+	if len(fields) > 0 {
+		log.WithFields(logrus.Fields{"message": msg}).Debug()
+	} else {
+		log.Debug(msg)
+	}
+}
+
+func Debugf(format string, args ...interface{}) {
+	log.Debugf(format, args...)
+}
+
+func Warn(msg string, fields ...interface{}) {
+	if len(fields) > 0 {
+		log.WithFields(logrus.Fields{"message": msg}).Warn()
+	} else {
+		log.Warn(msg)
+	}
+}
+
+func Warnf(format string, args ...interface{}) {
+	log.Warnf(format, args...)
+}
+
+func Error(msg string, fields ...interface{}) {
+	if len(fields) > 0 {
+		log.WithFields(logrus.Fields{"message": msg}).Error()
+	} else {
+		log.Error(msg)
+	}
+}
+
+func Errorf(format string, args ...interface{}) {
+	log.Errorf(format, args...)
+}
+
+func Fatal(msg string, fields ...interface{}) {
+	if len(fields) > 0 {
+		log.WithFields(logrus.Fields{"message": msg}).Fatal()
+	} else {
+		log.Fatal(msg)
+	}
+}
+
+func Fatalf(format string, args ...interface{}) {
+	log.Fatalf(format, args...)
+}
+
+func WithField(key string, value interface{}) *logrus.Entry {
+	return log.WithField(key, value)
+}

backend/pkg/logger/logger_test.go

@@ -0,0 +1,58 @@
+package logger
+
+import (
+	"testing"
+)
+
+func TestLoggerInitialization(t *testing.T) {
+	// Reset logger to initial state
+	log = nil
+	
+	// Initialize logger
+	Init()
+	
+	// Verify logger is initialized
+	if log == nil {
+		t.Fatal("Logger was not initialized")
+	}
+}
+
+func TestLoggerInfo(t *testing.T) {
+	Init()
+	
+	// Test Infof
+	Infof("Test info message with %s", "format")
+	
+	// Test Info
+	Info("Test info message")
+}
+
+func TestLoggerDebug(t *testing.T) {
+	Init()
+	
+	// Test Debugf
+	Debugf("Test debug message with %s", "format")
+	
+	// Test Debug
+	Debug("Test debug message")
+}
+
+func TestLoggerWarn(t *testing.T) {
+	Init()
+	
+	// Test Warnf
+	Warnf("Test warn message with %s", "format")
+	
+	// Test Warn
+	Warn("Test warn message")
+}
+
+func TestLoggerError(t *testing.T) {
+	Init()
+	
+	// Test Errorf
+	Errorf("Test error message with %s", "format")
+	
+	// Test Error
+	Error("Test error message")
+}