feat: implement WYSIWYG markdown editor

Add complete markdown editor with Go backend and React/TypeScript frontend.

Backend:
- Cobra CLI with configurable host, port, data-dir, static-dir flags
- REST API for CRUD operations on markdown files (GET, POST, PUT, DELETE)
- File storage with flat .md structure
- Comprehensive Logrus logging for all operations
- Static asset serving for frontend

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Live markdown editor with GFM preview (react-markdown)
- File management UI (list, create, open, save, delete)
- Theme system (Light/Dark/System) with localStorage persistence
- Responsive design (320px - 1920px+)

Testing:
- 6 backend tests covering CRUD round-trip, validation, error handling
- 19 frontend tests covering API, theme system, and UI components
- All tests passing with single 'make test' command

Build:
- Frontend compiles to optimized assets in dist/
- Backend can serve frontend via --static-dir flag

backend/internal/api/handlers.go

@@ -0,0 +1,171 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/evanreichard/markdown-editor/internal/logging"
+	"github.com/evanreichard/markdown-editor/internal/storage"
+)
+
+// Handlers contains the HTTP handlers
+type Handlers struct {
+	storage *storage.Storage
+}
+
+// New creates a new Handlers instance
+func New(s *storage.Storage) *Handlers {
+	logging.Logger.Info("API handlers initialized")
+	return &Handlers{storage: s}
+}
+
+// ErrorResponse represents an error response
+type ErrorResponse struct {
+	Error string `json:"error"`
+}
+
+// sendError sends a JSON error response
+func (h *Handlers) sendError(w http.ResponseWriter, r *http.Request, statusCode int, err error) {
+	logging.Logger.WithFields(map[string]interface{}{
+		"path":        r.URL.Path,
+		"status":      statusCode,
+		"error":       err.Error(),
+	}).Warn("API error")
+	
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(statusCode)
+	json.NewEncoder(w).Encode(ErrorResponse{Error: err.Error()})
+}
+
+// ListFiles handles GET /api/files
+func (h *Handlers) ListFiles(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		h.sendError(w, r, http.StatusMethodNotAllowed, fmt.Errorf("method not allowed"))
+		return
+	}
+
+	files, err := h.storage.List()
+	if err != nil {
+		h.sendError(w, r, http.StatusInternalServerError, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(files)
+}
+
+// GetFile handles GET /api/files/:name
+func (h *Handlers) GetFile(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodGet {
+		h.sendError(w, r, http.StatusMethodNotAllowed, fmt.Errorf("method not allowed"))
+		return
+	}
+
+	name := strings.TrimPrefix(r.URL.Path, "/api/files/")
+	if name == "" {
+		h.sendError(w, r, http.StatusBadRequest, fmt.Errorf("file name required"))
+		return
+	}
+
+	file, err := h.storage.Get(name)
+	if err != nil {
+		if err == storage.ErrFileNotFound {
+			h.sendError(w, r, http.StatusNotFound, err)
+			return
+		}
+		h.sendError(w, r, http.StatusBadRequest, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(file)
+}
+
+// CreateFile handles POST /api/files
+func (h *Handlers) CreateFile(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPost {
+		h.sendError(w, r, http.StatusMethodNotAllowed, fmt.Errorf("method not allowed"))
+		return
+	}
+
+	var file storage.File
+	if err := json.NewDecoder(r.Body).Decode(&file); err != nil {
+		h.sendError(w, r, http.StatusBadRequest, fmt.Errorf("invalid request body: %w", err))
+		return
+	}
+
+	result, err := h.storage.Create(file.Name, file.Content)
+	if err != nil {
+		if err == storage.ErrInvalidName {
+			h.sendError(w, r, http.StatusBadRequest, err)
+			return
+		}
+		h.sendError(w, r, http.StatusConflict, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusCreated)
+	json.NewEncoder(w).Encode(result)
+}
+
+// UpdateFile handles PUT /api/files/:name
+func (h *Handlers) UpdateFile(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodPut {
+		h.sendError(w, r, http.StatusMethodNotAllowed, fmt.Errorf("method not allowed"))
+		return
+	}
+
+	name := strings.TrimPrefix(r.URL.Path, "/api/files/")
+	if name == "" {
+		h.sendError(w, r, http.StatusBadRequest, fmt.Errorf("file name required"))
+		return
+	}
+
+	var file storage.File
+	if err := json.NewDecoder(r.Body).Decode(&file); err != nil {
+		h.sendError(w, r, http.StatusBadRequest, fmt.Errorf("invalid request body: %w", err))
+		return
+	}
+
+	result, err := h.storage.Update(name, file.Content)
+	if err != nil {
+		if err == storage.ErrFileNotFound {
+			h.sendError(w, r, http.StatusNotFound, err)
+			return
+		}
+		h.sendError(w, r, http.StatusBadRequest, err)
+		return
+	}
+
+	w.Header().Set("Content-Type", "application/json")
+	json.NewEncoder(w).Encode(result)
+}
+
+// DeleteFile handles DELETE /api/files/:name
+func (h *Handlers) DeleteFile(w http.ResponseWriter, r *http.Request) {
+	if r.Method != http.MethodDelete {
+		h.sendError(w, r, http.StatusMethodNotAllowed, fmt.Errorf("method not allowed"))
+		return
+	}
+
+	name := strings.TrimPrefix(r.URL.Path, "/api/files/")
+	if name == "" {
+		h.sendError(w, r, http.StatusBadRequest, fmt.Errorf("file name required"))
+		return
+	}
+
+	err := h.storage.Delete(name)
+	if err != nil {
+		if err == storage.ErrFileNotFound {
+			h.sendError(w, r, http.StatusNotFound, err)
+			return
+		}
+		h.sendError(w, r, http.StatusBadRequest, err)
+		return
+	}
+
+	w.WriteHeader(http.StatusNoContent)
+}