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/server/server.go

@@ -0,0 +1,172 @@
+package server
+
+import (
+	"encoding/json"
+	"fmt"
+	"net"
+	"net/http"
+
+	"github.com/sirupsen/logrus"
+	"markdown-editor-backend/handlers"
+	"markdown-editor-backend/storage"
+)
+
+// Server represents the HTTP server
+type Server struct {
+	dataDir    string
+	host       string
+	port       int
+	log        *logrus.Logger
+	storage    *storage.Storage
+	handler    *handlers.Handler
+	staticPath string
+	listener   net.Listener
+}
+
+// New creates a new Server instance
+func New(dataDir, host string, port int, log *logrus.Logger) *Server {
+	store := storage.New(dataDir, log)
+	handler := handlers.New(store, log)
+	return &Server{
+		dataDir:    dataDir,
+		host:       host,
+		port:       port,
+		log:        log,
+		storage:    store,
+		handler:    handler,
+		staticPath: "./static",
+	}
+}
+
+// SetStaticPath sets the path for static files (used for embedding frontend)
+func (s *Server) SetStaticPath(path string) {
+	s.staticPath = path
+}
+
+// Start initializes storage and starts the HTTP server
+func (s *Server) Start() error {
+	if err := s.storage.Init(); err != nil {
+		return err
+	}
+
+	mux := http.NewServeMux()
+
+	// API routes
+	mux.HandleFunc("/api/files", s.handler.FilesHandler)
+	mux.HandleFunc("/api/files/", s.handler.FileHandler)
+
+	// Health check
+	mux.HandleFunc("/api/health", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
+	})
+
+	// Static file serving - serve frontend build
+	fs := http.FileServer(http.Dir(s.staticPath))
+	mux.Handle("/", fs)
+
+	// CORS middleware
+	handler := s.corsMiddleware(mux)
+
+	addr := fmt.Sprintf("%s:%d", s.host, s.port)
+	s.log.WithField("address", addr).Info("Server starting")
+
+	listener, err := net.Listen("tcp", addr)
+	if err != nil {
+		return err
+	}
+	s.listener = listener
+
+	return http.Serve(listener, handler)
+}
+
+// StartAsync starts the server asynchronously and returns the actual address
+func (s *Server) StartAsync() (string, error) {
+	if err := s.storage.Init(); err != nil {
+		return "", err
+	}
+
+	mux := http.NewServeMux()
+
+	// API routes
+	mux.HandleFunc("/api/files", s.handler.FilesHandler)
+	mux.HandleFunc("/api/files/", s.handler.FileHandler)
+
+	// Health check
+	mux.HandleFunc("/api/health", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
+	})
+
+	// Static file serving - serve frontend build
+	fs := http.FileServer(http.Dir(s.staticPath))
+	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
+		if len(r.URL.Path) >= 4 && r.URL.Path[:4] == "/api" {
+			return
+		}
+		fs.ServeHTTP(w, r)
+	})
+
+	// CORS middleware
+	handler := s.corsMiddleware(mux)
+
+	addr := fmt.Sprintf("%s:%d", s.host, s.port)
+	if s.port == 0 {
+		addr = fmt.Sprintf("%s:0", s.host)
+	}
+
+	listener, err := net.Listen("tcp", addr)
+	if err != nil {
+		return "", err
+	}
+	s.listener = listener
+
+	actualAddr := listener.Addr().String()
+	s.log.WithField("address", actualAddr).Info("Server starting")
+
+	go http.Serve(listener, handler)
+
+	return actualAddr, nil
+}
+
+// Stop stops the server
+func (s *Server) Stop() error {
+	if s.listener != nil {
+		return s.listener.Close()
+	}
+	return nil
+}
+
+// corsMiddleware adds CORS headers for frontend development
+func (s *Server) corsMiddleware(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Access-Control-Allow-Origin", "*")
+		w.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
+		w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
+
+		if r.Method == "OPTIONS" {
+			w.WriteHeader(http.StatusOK)
+			return
+		}
+
+		next.ServeHTTP(w, r)
+	})
+}
+
+// GetStorage returns the storage instance (for testing)
+func (s *Server) GetStorage() *storage.Storage {
+	return s.storage
+}
+
+// GetStaticPath returns the static files path (for testing)
+func (s *Server) GetStaticPath() string {
+	return s.staticPath
+}
+
+// GetAddr returns the server address (for testing)
+func (s *Server) GetAddr() string {
+	if s.listener != nil {
+		return s.listener.Addr().String()
+	}
+	return fmt.Sprintf("%s:%d", s.host, s.port)
+}