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/handlers/handlers.go

@@ -0,0 +1,176 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+	"strings"
+
+	"github.com/sirupsen/logrus"
+	"markdown-editor-backend/storage"
+)
+
+// Handler contains HTTP handlers for the API
+type Handler struct {
+	storage *storage.Storage
+	log     *logrus.Logger
+}
+
+// New creates a new Handler instance
+func New(storage *storage.Storage, log *logrus.Logger) *Handler {
+	return &Handler{
+		storage: storage,
+		log:     log,
+	}
+}
+
+// FileRequest represents a request body for file operations
+type FileRequest struct {
+	Name    string `json:"name"`
+	Content string `json:"content"`
+}
+
+// FileResponse represents a file response
+type FileResponse struct {
+	Name    string `json:"name"`
+	Content string `json:"content,omitempty"`
+}
+
+// ErrorResponse represents an error response
+type ErrorResponse struct {
+	Error string `json:"error"`
+}
+
+// FilesHandler handles list and create operations
+func (h *Handler) FilesHandler(w http.ResponseWriter, r *http.Request) {
+	switch r.Method {
+	case http.MethodGet:
+		h.listFiles(w, r)
+	case http.MethodPost:
+		h.createFile(w, r)
+	default:
+		h.sendError(w, http.StatusMethodNotAllowed, "method not allowed")
+	}
+}
+
+// FileHandler handles get, update, and delete operations for a specific file
+func (h *Handler) FileHandler(w http.ResponseWriter, r *http.Request) {
+	// Extract filename from URL path
+	path := strings.TrimPrefix(r.URL.Path, "/api/files/")
+	if path == "" {
+		h.sendError(w, http.StatusBadRequest, "filename required")
+		return
+	}
+
+	name := strings.TrimSuffix(path, ".md")
+
+	switch r.Method {
+	case http.MethodGet:
+		h.getFile(w, r, name)
+	case http.MethodPut:
+		h.updateFile(w, r, name)
+	case http.MethodDelete:
+		h.deleteFile(w, r, name)
+	default:
+		h.sendError(w, http.StatusMethodNotAllowed, "method not allowed")
+	}
+}
+
+func (h *Handler) listFiles(w http.ResponseWriter, r *http.Request) {
+	h.log.Debug("Handling list files request")
+
+	files, err := h.storage.List()
+	if err != nil {
+		h.log.WithError(err).Error("Failed to list files")
+		h.sendError(w, http.StatusInternalServerError, "failed to list files")
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(files)
+}
+
+func (h *Handler) createFile(w http.ResponseWriter, r *http.Request) {
+	h.log.Debug("Handling create file request")
+
+	var req FileRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		h.log.WithError(err).Warn("Invalid request body")
+		h.sendError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if req.Name == "" {
+		h.sendError(w, http.StatusBadRequest, "name is required")
+		return
+	}
+
+	if err := h.storage.Save(req.Name, req.Content); err != nil {
+		h.log.WithError(err).Error("Failed to create file")
+		h.sendError(w, http.StatusInternalServerError, "failed to create file")
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusCreated)
+	json.NewEncoder(w).Encode(FileResponse{Name: req.Name, Content: req.Content})
+}
+
+func (h *Handler) getFile(w http.ResponseWriter, r *http.Request, name string) {
+	h.log.WithField("name", name).Debug("Handling get file request")
+
+	content, err := h.storage.Get(name)
+	if err != nil {
+		if err.Error() == "file not found" {
+			h.sendError(w, http.StatusNotFound, "file not found")
+			return
+		}
+		h.log.WithError(err).Error("Failed to get file")
+		h.sendError(w, http.StatusInternalServerError, "failed to get file")
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(FileResponse{Name: name, Content: content})
+}
+
+func (h *Handler) updateFile(w http.ResponseWriter, r *http.Request, name string) {
+	h.log.WithField("name", name).Debug("Handling update file request")
+
+	var req FileRequest
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		h.log.WithError(err).Warn("Invalid request body")
+		h.sendError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if err := h.storage.Save(name, req.Content); err != nil {
+		h.log.WithError(err).Error("Failed to update file")
+		h.sendError(w, http.StatusInternalServerError, "failed to update file")
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(FileResponse{Name: name, Content: req.Content})
+}
+
+func (h *Handler) deleteFile(w http.ResponseWriter, r *http.Request, name string) {
+	h.log.WithField("name", name).Debug("Handling delete file request")
+
+	if err := h.storage.Delete(name); err != nil {
+		if err.Error() == "file not found" {
+			h.sendError(w, http.StatusNotFound, "file not found")
+			return
+		}
+		h.log.WithError(err).Error("Failed to delete file")
+		h.sendError(w, http.StatusInternalServerError, "failed to delete file")
+		return
+	}
+
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (h *Handler) sendError(w http.ResponseWriter, status int, message string) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(ErrorResponse{Error: message})
+}