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/storage/storage.go

@@ -0,0 +1,173 @@
+package storage
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/evanreichard/markdown-editor/internal/logging"
+)
+
+var (
+	ErrFileNotFound = errors.New("file not found")
+	ErrInvalidName  = errors.New("invalid file name")
+)
+
+// File represents a markdown file
+type File struct {
+	Name     string `json:"name"`
+	Content  string `json:"content"`
+	Modified int64  `json:"modified"`
+}
+
+// Storage handles file operations
+type Storage struct {
+	dataDir string
+}
+
+// New creates a new Storage instance
+func New(dataDir string) (*Storage, error) {
+	if err := os.MkdirAll(dataDir, 0755); err != nil {
+		return nil, fmt.Errorf("failed to create data directory: %w", err)
+	}
+	logging.Logger.WithField("data_dir", dataDir).Info("Storage initialized")
+	return &Storage{dataDir: dataDir}, nil
+}
+
+// List returns all markdown files
+func (s *Storage) List() ([]*File, error) {
+	entries, err := os.ReadDir(s.dataDir)
+	if err != nil {
+		logging.Logger.WithError(err).Error("Failed to list files")
+		return nil, fmt.Errorf("failed to read directory: %w", err)
+	}
+
+	var files []*File
+	for _, entry := range entries {
+		if entry.IsDir() || !strings.HasSuffix(entry.Name(), ".md") {
+			continue
+		}
+		
+		info, err := entry.Info()
+		if err != nil {
+			continue
+		}
+		
+		files = append(files, &File{
+			Name:     entry.Name(),
+			Modified: info.ModTime().Unix(),
+		})
+	}
+	
+	logging.Logger.WithField("count", len(files)).Info("Listed files")
+	return files, nil
+}
+
+// Get reads a markdown file
+func (s *Storage) Get(name string) (*File, error) {
+	if !s.isValidName(name) {
+		logging.Logger.WithField("name", name).Warn("Invalid file name")
+		return nil, ErrInvalidName
+	}
+
+	path := filepath.Join(s.dataDir, name)
+	content, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			logging.Logger.WithField("name", name).Warn("File not found")
+			return nil, ErrFileNotFound
+		}
+		logging.Logger.WithError(err).Error("Failed to read file")
+		return nil, fmt.Errorf("failed to read file: %w", err)
+	}
+
+	info, err := os.Stat(path)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get file info: %w", err)
+	}
+
+	logging.Logger.WithField("name", name).Info("File retrieved")
+	return &File{
+		Name:     name,
+		Content:  string(content),
+		Modified: info.ModTime().Unix(),
+	}, nil
+}
+
+// Create creates a new markdown file
+func (s *Storage) Create(name, content string) (*File, error) {
+	if !s.isValidName(name) {
+		logging.Logger.WithField("name", name).Warn("Invalid file name for create")
+		return nil, ErrInvalidName
+	}
+
+	path := filepath.Join(s.dataDir, name)
+	if _, err := os.Stat(path); err == nil {
+		logging.Logger.WithField("name", name).Warn("File already exists")
+		return nil, fmt.Errorf("file already exists")
+	}
+
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		logging.Logger.WithError(err).Error("Failed to create file")
+		return nil, fmt.Errorf("failed to write file: %w", err)
+	}
+
+	logging.Logger.WithField("name", name).Info("File created")
+	return s.Get(name)
+}
+
+// Update updates an existing markdown file
+func (s *Storage) Update(name, content string) (*File, error) {
+	if !s.isValidName(name) {
+		logging.Logger.WithField("name", name).Warn("Invalid file name for update")
+		return nil, ErrInvalidName
+	}
+
+	path := filepath.Join(s.dataDir, name)
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		logging.Logger.WithField("name", name).Warn("File not found for update")
+		return nil, ErrFileNotFound
+	}
+
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		logging.Logger.WithError(err).Error("Failed to update file")
+		return nil, fmt.Errorf("failed to write file: %w", err)
+	}
+
+	logging.Logger.WithField("name", name).Info("File updated")
+	return s.Get(name)
+}
+
+// Delete removes a markdown file
+func (s *Storage) Delete(name string) error {
+	if !s.isValidName(name) {
+		logging.Logger.WithField("name", name).Warn("Invalid file name for delete")
+		return ErrInvalidName
+	}
+
+	path := filepath.Join(s.dataDir, name)
+	if err := os.Remove(path); err != nil {
+		if os.IsNotExist(err) {
+			logging.Logger.WithField("name", name).Warn("File not found for delete")
+			return ErrFileNotFound
+		}
+		logging.Logger.WithError(err).Error("Failed to delete file")
+		return fmt.Errorf("failed to delete file: %w", err)
+	}
+
+	logging.Logger.WithField("name", name).Info("File deleted")
+	return nil
+}
+
+// isValidName checks if the file name is valid
+func (s *Storage) isValidName(name string) bool {
+	if name == "" || strings.Contains(name, "..") || strings.Contains(name, "/") {
+		return false
+	}
+	if !strings.HasSuffix(name, ".md") {
+		return false
+	}
+	return true
+}