feat: implement WYSIWYG markdown editor

Add full-stack markdown editor with Go backend and React frontend.

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- REST API for markdown file CRUD operations
- File storage with flat directory structure
- logrus logging for all operations
- Static file serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend:
- React + TypeScript + Vite + Tailwind CSS
- Live markdown preview with marked (GFM)
- File management: list, create, open, save, delete
- Theme system: Dark/Light/System with persistence
- Responsive design (320px to 1920px)
- Component tests for Editor, Preview, Sidebar

Build:
- Makefile for build, test, and run automation
- Single command testing (make test)

Closes SPEC.md requirements

backend/main.go

@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/sirupsen/logrus"
+	"github.com/spf13/cobra"
+	"markdown-editor-backend/server"
+)
+
+var (
+	dataDir string
+	port    int
+	host    string
+	log     = logrus.New()
+)
+
+var rootCmd = &cobra.Command{
+	Use:   "markdown-editor-backend",
+	Short: "WYSIWYG Markdown Editor Backend",
+	Long:  `A Go backend server for the WYSIWYG Markdown Editor with CRUD operations for markdown files.`,
+	Run: func(cmd *cobra.Command, args []string) {
+		log.SetFormatter(&logrus.TextFormatter{
+			FullTimestamp: true,
+		})
+		log.SetLevel(logrus.InfoLevel)
+
+		log.WithFields(logrus.Fields{
+			"dataDir": dataDir,
+			"host":    host,
+			"port":    port,
+		}).Info("Starting server")
+
+		srv := server.New(dataDir, host, port, log)
+		if err := srv.Start(); err != nil {
+			log.WithError(err).Fatal("Server failed to start")
+		}
+	},
+}
+
+func init() {
+	rootCmd.Flags().StringVar(&dataDir, "data-dir", "./data", "Storage path for markdown files")
+	rootCmd.Flags().IntVar(&port, "port", 8080, "Server port")
+	rootCmd.Flags().StringVar(&host, "host", "127.0.0.1", "Bind address")
+}
+
+func main() {
+	if err := rootCmd.Execute(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}