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/storage/storage.go

@@ -0,0 +1,150 @@
+package storage
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync"
+
+	"github.com/sirupsen/logrus"
+)
+
+// Storage handles file operations for markdown files
+type Storage struct {
+	dataDir string
+	mu      sync.RWMutex
+	log     *logrus.Logger
+}
+
+// New creates a new Storage instance
+func New(dataDir string, log *logrus.Logger) *Storage {
+	return &Storage{
+		dataDir: dataDir,
+		log:     log,
+	}
+}
+
+// Init ensures the data directory exists
+func (s *Storage) Init() error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.log.WithField("dataDir", s.dataDir).Info("Initializing storage")
+
+	if err := os.MkdirAll(s.dataDir, 0755); err != nil {
+		s.log.WithError(err).Error("Failed to create data directory")
+		return fmt.Errorf("failed to create data directory: %w", err)
+	}
+	return nil
+}
+
+// List returns all markdown files in the data directory
+func (s *Storage) List() ([]string, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	s.log.Debug("Listing all files")
+
+	entries, err := os.ReadDir(s.dataDir)
+	if err != nil {
+		s.log.WithError(err).Error("Failed to read data directory")
+		return nil, fmt.Errorf("failed to read data directory: %w", err)
+	}
+
+	var files []string
+	for _, entry := range entries {
+		if !entry.IsDir() && strings.HasSuffix(entry.Name(), ".md") {
+			files = append(files, strings.TrimSuffix(entry.Name(), ".md"))
+		}
+	}
+
+	s.log.WithField("count", len(files)).Debug("Listed files")
+	return files, nil
+}
+
+// Get reads a markdown file by name
+func (s *Storage) Get(name string) (string, error) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	s.log.WithField("name", name).Debug("Getting file")
+
+	if !isValidFilename(name) {
+		s.log.WithField("name", name).Warn("Invalid filename")
+		return "", fmt.Errorf("invalid filename")
+	}
+
+	path := filepath.Join(s.dataDir, name+".md")
+	content, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			s.log.WithField("name", name).Warn("File not found")
+			return "", fmt.Errorf("file not found")
+		}
+		s.log.WithError(err).WithField("name", name).Error("Failed to read file")
+		return "", fmt.Errorf("failed to read file: %w", err)
+	}
+
+	return string(content), nil
+}
+
+// Save writes content to a markdown file
+func (s *Storage) Save(name, content string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.log.WithField("name", name).Debug("Saving file")
+
+	if !isValidFilename(name) {
+		s.log.WithField("name", name).Warn("Invalid filename")
+		return fmt.Errorf("invalid filename")
+	}
+
+	path := filepath.Join(s.dataDir, name+".md")
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		s.log.WithError(err).WithField("name", name).Error("Failed to write file")
+		return fmt.Errorf("failed to write file: %w", err)
+	}
+
+	s.log.WithField("name", name).Info("File saved")
+	return nil
+}
+
+// Delete removes a markdown file
+func (s *Storage) Delete(name string) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	s.log.WithField("name", name).Debug("Deleting file")
+
+	if !isValidFilename(name) {
+		s.log.WithField("name", name).Warn("Invalid filename")
+		return fmt.Errorf("invalid filename")
+	}
+
+	path := filepath.Join(s.dataDir, name+".md")
+	if err := os.Remove(path); err != nil {
+		if os.IsNotExist(err) {
+			s.log.WithField("name", name).Warn("File not found for deletion")
+			return fmt.Errorf("file not found")
+		}
+		s.log.WithError(err).WithField("name", name).Error("Failed to delete file")
+		return fmt.Errorf("failed to delete file: %w", err)
+	}
+
+	s.log.WithField("name", name).Info("File deleted")
+	return nil
+}
+
+// isValidFilename checks if a filename is safe to use
+func isValidFilename(name string) bool {
+	if name == "" {
+		return false
+	}
+	// Prevent path traversal
+	if strings.Contains(name, "/") || strings.Contains(name, "\\") || strings.Contains(name, "..") {
+		return false
+	}
+	return true
+}