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/cmd/server/main.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/markdown-editor/internal/api"
+	"github.com/markdown-editor/pkg/logger"
+	"github.com/spf13/cobra"
+)
+
+var (
+	dataDir string
+	host    string
+	port    int
+)
+
+func main() {
+	rootCmd := &cobra.Command{
+		Use:   "server",
+		Short: "Markdown Editor Server",
+		Run:   runServer,
+	}
+
+	rootCmd.Flags().StringVar(&dataDir, "data-dir", "./data", "Storage path for markdown files")
+	rootCmd.Flags().StringVar(&host, "host", "127.0.0.1", "Bind address")
+	rootCmd.Flags().IntVar(&port, "port", 8080, "Server port")
+
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}
+
+func runServer(cmd *cobra.Command, args []string) {
+	// Initialize logger
+	logger.Init()
+
+	logger.Info("Starting Markdown Editor Server")
+	logger.Infof("Data directory: %s", dataDir)
+	logger.Infof("Server will bind to %s:%d", host, port)
+
+	// Initialize API server
+	svr, err := api.NewServer(dataDir, host, port)
+	if err != nil {
+		logger.Fatalf("Failed to initialize server: %v", err)
+	}
+
+	// Setup signal handling for graceful shutdown
+	ctx, cancel := signal.NotifyContext(cmd.Context(), syscall.SIGINT, syscall.SIGTERM)
+	defer cancel()
+
+	// Start server in a goroutine
+	errChan := make(chan error, 1)
+	go func() {
+		logger.Infof("Server listening on %s:%d", host, port)
+		errChan <- svr.Start()
+	}()
+
+	// Wait for shutdown signal or error
+	select {
+	case <-ctx.Done():
+		logger.Info("Shutdown signal received")
+	case err := <-errChan:
+		if err != nil {
+			logger.Fatalf("Server error: %v", err)
+		}
+	}
+
+	logger.Info("Server stopped gracefully")
+}