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_test.go

@@ -0,0 +1,168 @@
+package storage
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestStorage_ListFiles(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	store, err := NewStorage(tempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create test files
+	files := []string{"file1.md", "file2.md", "file3.txt"}
+	for _, name := range files {
+		err = os.WriteFile(filepath.Join(tempDir, name), []byte("# Content"), 0644)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	result, err := store.ListFiles()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if len(result) != 2 {
+		t.Errorf("Expected 2 .md files, got %d", len(result))
+	}
+
+	// Verify only .md files are listed
+	for _, name := range result {
+		if !IsMarkdownFile(name) {
+			t.Errorf("Non-markdown file found: %s", name)
+		}
+	}
+}
+
+func TestStorage_CreateFile(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	store, err := NewStorage(tempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = store.CreateFile("test.md", "# Test Content")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	content, err := os.ReadFile(filepath.Join(tempDir, "test.md"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(content) != "# Test Content" {
+		t.Errorf("Expected '# Test Content', got '%s'", string(content))
+	}
+}
+
+func TestStorage_UpdateFile(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	store, err := NewStorage(tempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create file
+	err = store.CreateFile("test.md", "# Original Content")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Update file
+	err = store.UpdateFile("test.md", "# Updated Content")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	content, err := os.ReadFile(filepath.Join(tempDir, "test.md"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if string(content) != "# Updated Content" {
+		t.Errorf("Expected '# Updated Content', got '%s'", string(content))
+	}
+}
+
+func TestStorage_DeleteFile(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	store, err := NewStorage(tempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create file
+	err = store.CreateFile("test.md", "# Test Content")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Delete file
+	err = store.DeleteFile("test.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = os.Stat(filepath.Join(tempDir, "test.md"))
+	if !os.IsNotExist(err) {
+		t.Error("File was not deleted")
+	}
+}
+
+func TestStorage_GetFile(t *testing.T) {
+	tempDir, err := os.MkdirTemp("", "test-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(tempDir)
+
+	store, err := NewStorage(tempDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Create file
+	err = store.CreateFile("test.md", "# Test Content")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	file, err := store.GetFile("test.md")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if file.Content != "# Test Content" {
+		t.Errorf("Expected '# Test Content', got '%s'", file.Content)
+	}
+}
+
+func IsMarkdownFile(filename string) bool {
+	return filepath.Ext(filename) == ".md"
+}