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/internal/api/server.go

@@ -0,0 +1,195 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"path/filepath"
+	"strings"
+
+	"github.com/markdown-editor/internal/storage"
+	"github.com/markdown-editor/pkg/logger"
+)
+
+type ErrorResponse struct {
+	Error string `json:"error"`
+}
+
+type Server struct {
+	storage *storage.Storage
+	host    string
+	port    int
+}
+
+func NewServer(dataDir, host string, port int) (*Server, error) {
+	return &Server{
+		storage: storage.NewStorage(dataDir),
+		host:    host,
+		port:    port,
+	}, nil
+}
+
+func (s *Server) Start() error {
+	mux := http.NewServeMux()
+
+	// API routes
+	mux.HandleFunc("/api/files", s.handleFiles)
+	mux.HandleFunc("/api/files/", s.handleFiles)
+
+	// Static files
+	mux.Handle("/static/", http.StripPrefix("/static/", http.FileServer(http.Dir("./static"))))
+
+	// Frontend SPA fallback
+	mux.HandleFunc("/", s.handleFrontend)
+
+	addr := fmt.Sprintf("%s:%d", s.host, s.port)
+	logger.Infof("Starting server on %s", addr)
+
+	return http.ListenAndServe(addr, mux)
+}
+
+func (s *Server) handleFiles(w http.ResponseWriter, r *http.Request) {
+	switch r.Method {
+	case http.MethodGet:
+		s.handleGetFiles(w, r)
+	case http.MethodPost:
+		s.handleCreateFile(w, r)
+	case http.MethodPut:
+		s.handleUpdateFile(w, r)
+	case http.MethodDelete:
+		s.handleDeleteFile(w, r)
+	default:
+		s.sendError(w, http.StatusMethodNotAllowed, "method not allowed")
+	}
+}
+
+func (s *Server) handleGetFiles(w http.ResponseWriter, r *http.Request) {
+	// Check if URL includes a filename (e.g., /api/files/test.md)
+	filename := filepath.Base(r.URL.Path)
+	if filename != "" && r.URL.Path != "/api/files" {
+		// Get specific file
+		content, err := s.storage.GetFile(filename)
+		if err != nil {
+			s.sendError(w, http.StatusNotFound, err.Error())
+			return
+		}
+		w.Header().Set("Content-Type", "text/markdown")
+		w.Write([]byte(content))
+		return
+	}
+
+	// List all files
+	files, err := s.storage.ListFiles()
+	if err != nil {
+		s.sendError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.Write([]byte{'['})
+	for i, file := range files {
+		if i > 0 {
+			w.Write([]byte{','})
+		}
+		w.Write([]byte{'"'})
+		w.Write([]byte(file))
+		w.Write([]byte{'"'})
+	}
+	w.Write([]byte{']'})
+}
+
+func (s *Server) handleCreateFile(w http.ResponseWriter, r *http.Request) {
+	var req struct {
+		Content string `json:"content"`
+		Name    string `json:"name"`
+	}
+
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		s.sendError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if req.Name == "" {
+		s.sendError(w, http.StatusBadRequest, "filename is required")
+		return
+	}
+
+	if !strings.HasSuffix(req.Name, ".md") {
+		s.sendError(w, http.StatusBadRequest, "filename must end with .md")
+		return
+	}
+
+	if err := s.storage.SaveFile(req.Name, req.Content); err != nil {
+		s.sendError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	logger.Infof("Created file: %s", req.Name)
+	w.WriteHeader(http.StatusCreated)
+}
+
+func (s *Server) handleUpdateFile(w http.ResponseWriter, r *http.Request) {
+	filename := filepath.Base(r.URL.Path)
+
+	if filename == "" {
+		s.sendError(w, http.StatusBadRequest, "filename is required")
+		return
+	}
+
+	if !strings.HasSuffix(filename, ".md") {
+		s.sendError(w, http.StatusBadRequest, "filename must end with .md")
+		return
+	}
+
+	var req struct {
+		Content string `json:"content"`
+	}
+
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		s.sendError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if err := s.storage.SaveFile(filename, req.Content); err != nil {
+		s.sendError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	logger.Infof("Updated file: %s", filename)
+}
+
+func (s *Server) handleDeleteFile(w http.ResponseWriter, r *http.Request) {
+	filename := filepath.Base(r.URL.Path)
+
+	if filename == "" {
+		s.sendError(w, http.StatusBadRequest, "filename is required")
+		return
+	}
+
+	if !strings.HasSuffix(filename, ".md") {
+		s.sendError(w, http.StatusBadRequest, "filename must end with .md")
+		return
+	}
+
+	if err := s.storage.DeleteFile(filename); err != nil {
+		s.sendError(w, http.StatusInternalServerError, err.Error())
+		return
+	}
+
+	logger.Infof("Deleted file: %s", filename)
+}
+
+func (s *Server) handleFrontend(w http.ResponseWriter, r *http.Request) {
+	// Serve the index.html for SPA
+	http.ServeFile(w, r, "./static/index.html")
+}
+
+func (s *Server) sendError(w http.ResponseWriter, statusCode int, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(statusCode)
+	json.NewEncoder(w).Encode(ErrorResponse{Error: message})
+}
+
+func (s *Server) ServeStaticFiles(w http.ResponseWriter, r *http.Request) {
+	http.StripPrefix("/static/", http.FileServer(http.Dir("./static"))).ServeHTTP(w, r)
+}