Initial commit: WYSIWYG Markdown Editor - Go backend + React/TypeScript frontend with Tailwind CSS

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- Gin HTTP server with REST API for markdown CRUD operations
- File storage on disk (.md files only)
- Comprehensive logrus logging
- Backend tests with CRUD round-trip verification

Frontend:
- React 18 + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview (react-markdown + remark-gfm)
- File management UI (list, create, open, save, delete)
- Theme switcher with Dark/Light/System modes
- Responsive design
- Frontend tests with vitest

Testing:
- All backend tests pass (go test ./...)
- All frontend tests pass (npm test)

internal/storage/storage.go

@@ -0,0 +1,134 @@
+package storage
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+
+	"github.com/sirupsen/logrus"
+	"markdown-editor/internal/logger"
+)
+
+var (
+	allowedExt = regexp.MustCompile(`(?i)\.md$`)
+)
+
+type Storage struct {
+	dataDir string
+	log     *logrus.Logger
+}
+
+func NewStorage(dataDir string) (*Storage, error) {
+	log := logger.GetLogger()
+
+	// Create data directory if it doesn't exist
+	if err := os.MkdirAll(dataDir, 0755); err != nil {
+		return nil, fmt.Errorf("failed to create data directory: %w", err)
+	}
+
+	return &Storage{
+		dataDir: dataDir,
+		log:     log,
+	}, nil
+}
+
+// File represents a markdown file
+type File struct {
+	Name    string `json:"name"`
+	Content string `json:"content"`
+}
+
+// ListFiles returns all .md files in the data directory
+func (s *Storage) ListFiles() ([]string, error) {
+	s.log.WithField("operation", "list_files").Info("Listing markdown files")
+
+	files := []string{}
+	err := filepath.WalkDir(s.dataDir, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if !d.IsDir() && allowedExt.MatchString(d.Name()) {
+			files = append(files, d.Name())
+		}
+		return nil
+	})
+	if err != nil {
+		return nil, fmt.Errorf("failed to walk directory: %w", err)
+	}
+	return files, nil
+}
+
+// GetFile reads a markdown file by name
+func (s *Storage) GetFile(name string) (*File, error) {
+	s.log.WithField("operation", "get_file").WithField("filename", name).Info("Getting file")
+
+	path := filepath.Join(s.dataDir, name)
+	if !allowedExt.MatchString(path) {
+		return nil, fmt.Errorf("invalid file extension: only .md files allowed")
+	}
+
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("file not found: %s", name)
+		}
+		return nil, fmt.Errorf("failed to read file: %w", err)
+	}
+
+	return &File{
+		Name:    name,
+		Content: string(data),
+	}, nil
+}
+
+// CreateFile creates a new markdown file
+func (s *Storage) CreateFile(name string, content string) error {
+	s.log.WithField("operation", "create_file").WithField("filename", name).Info("Creating file")
+
+	path := filepath.Join(s.dataDir, name)
+	if !allowedExt.MatchString(path) {
+		return fmt.Errorf("invalid file extension: only .md files allowed")
+	}
+
+	// Check if file already exists
+	if _, err := os.Stat(path); err == nil {
+		return fmt.Errorf("file already exists: %s", name)
+	}
+
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		return fmt.Errorf("failed to write file: %w", err)
+	}
+	return nil
+}
+
+// UpdateFile updates an existing markdown file
+func (s *Storage) UpdateFile(name string, content string) error {
+	s.log.WithField("operation", "update_file").WithField("filename", name).Info("Updating file")
+
+	path := filepath.Join(s.dataDir, name)
+	if !allowedExt.MatchString(path) {
+		return fmt.Errorf("invalid file extension: only .md files allowed")
+	}
+
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		return fmt.Errorf("failed to write file: %w", err)
+	}
+	return nil
+}
+
+// DeleteFile deletes a markdown file
+func (s *Storage) DeleteFile(name string) error {
+	s.log.WithField("operation", "delete_file").WithField("filename", name).Info("Deleting file")
+
+	path := filepath.Join(s.dataDir, name)
+	if !allowedExt.MatchString(path) {
+		return fmt.Errorf("invalid file extension: only .md files allowed")
+	}
+
+	if err := os.Remove(path); err != nil {
+		return fmt.Errorf("failed to delete file: %w", err)
+	}
+	return nil
+}