feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

backend/internal/api/server_test.go

@@ -0,0 +1,191 @@
+package api
+
+import (
+	"encoding/json"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/markdown-editor/internal/storage"
+	"github.com/markdown-editor/pkg/logger"
+)
+
+type bytesReader struct {
+	bytes []byte
+}
+
+func (r *bytesReader) Read(p []byte) (n int, err error) {
+	if len(r.bytes) == 0 {
+		return 0, io.EOF
+	}
+	n = copy(p, r.bytes)
+	r.bytes = r.bytes[n:]
+	return n, nil
+}
+
+func (r *bytesReader) Close() error {
+	return nil
+}
+
+func setupTestServer(t *testing.T) (*Server, string) {
+	// Initialize logger
+	logger.Init()
+
+	// Create temporary directory for test data
+	tempDir := t.TempDir()
+	
+	// Initialize storage
+	s := storage.NewStorage(tempDir)
+	
+	// Create server
+	server, err := NewServer(tempDir, "127.0.0.1", 0)
+	if err != nil {
+		t.Fatalf("Failed to create server: %v", err)
+	}
+	
+	// Get actual port assigned by ListenAndServe
+	router := http.NewServeMux()
+	router.HandleFunc("/api/files", server.handleFiles)
+	router.HandleFunc("/api/files/", server.handleFiles)
+	router.HandleFunc("/", server.handleFrontend)
+	
+	server.storage = s
+	server.port = 0 // 0 means assign any available port
+	
+	return server, tempDir
+}
+
+func TestHandleGetFiles(t *testing.T) {
+	server, dataDir := setupTestServer(t)
+	
+	// Create test files
+	content := "Test content"
+	testFiles := []string{"test1.md", "test2.md"}
+	
+	for _, filename := range testFiles {
+		path := filepath.Join(dataDir, filename)
+		if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+			t.Fatalf("Failed to create test file: %v", err)
+		}
+	}
+	
+	req := httptest.NewRequest("GET", "/api/files", nil)
+	w := httptest.NewRecorder()
+	
+	server.handleGetFiles(w, req)
+	
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status 200, got %d", w.Code)
+	}
+	
+	var files []string
+	if err := json.NewDecoder(w.Body).Decode(&files); err != nil {
+		t.Fatalf("Failed to decode response: %v", err)
+	}
+	
+	if len(files) != 2 {
+		t.Errorf("Expected 2 files, got %d", len(files))
+	}
+}
+
+func TestHandleCreateFile(t *testing.T) {
+	server, dataDir := setupTestServer(t)
+	
+	reqBody := map[string]string{
+		"content": "Test content",
+		"name":    "newfile.md",
+	}
+	reqBodyBytes, _ := json.Marshal(reqBody)
+	
+	req := httptest.NewRequest("POST", "/api/files", nil)
+	req.Header.Set("Content-Type", "application/json")
+	req.Body = &bytesReader{bytes: reqBodyBytes}
+	
+	w := httptest.NewRecorder()
+	server.handleCreateFile(w, req)
+	
+	if w.Code != http.StatusCreated {
+		t.Errorf("Expected status 201, got %d", w.Code)
+	}
+	
+	// Verify file was created
+	path := filepath.Join(dataDir, "newfile.md")
+	if _, err := os.Stat(path); os.IsNotExist(err) {
+		t.Error("File was not created")
+	}
+}
+
+func TestHandleUpdateFile(t *testing.T) {
+	server, dataDir := setupTestServer(t)
+	
+	// Create test file first
+	filename := "updatefile.md"
+	path := filepath.Join(dataDir, filename)
+	os.WriteFile(path, []byte("Original content"), 0644)
+	
+	reqBody := map[string]string{
+		"content": "Updated content",
+	}
+	reqBodyBytes, _ := json.Marshal(reqBody)
+	
+	req := httptest.NewRequest("PUT", "/api/files/"+filename, nil)
+	req.Header.Set("Content-Type", "application/json")
+	req.Body = &bytesReader{bytes: reqBodyBytes}
+	
+	w := httptest.NewRecorder()
+	server.handleUpdateFile(w, req)
+	
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status 200, got %d", w.Code)
+	}
+	
+	// Verify file content was updated
+	newContent, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("Failed to read file: %v", err)
+	}
+	if string(newContent) != "Updated content" {
+		t.Error("File content was not updated correctly")
+	}
+}
+
+func TestHandleDeleteFile(t *testing.T) {
+	server, dataDir := setupTestServer(t)
+	
+	// Create test file
+	filename := "deletefile.md"
+	path := filepath.Join(dataDir, filename)
+	os.WriteFile(path, []byte("Test content"), 0644)
+	
+	req := httptest.NewRequest("DELETE", "/api/files/"+filename, nil)
+	w := httptest.NewRecorder()
+	
+	server.handleDeleteFile(w, req)
+	
+	if w.Code != http.StatusOK {
+		t.Errorf("Expected status 200, got %d", w.Code)
+	}
+	
+	// Verify file was deleted
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Error("File was not deleted")
+	}
+}
+
+func TestHandleStaticFiles(t *testing.T) {
+	server, _ := setupTestServer(t)
+	
+	// Try to serve static file
+	req := httptest.NewRequest("GET", "/static/index.html", nil)
+	w := httptest.NewRecorder()
+	
+	server.handleFrontend(w, req)
+	
+	// Should return 301 redirect or 200 for index.html
+	if w.Code != http.StatusOK && w.Code != http.StatusMovedPermanently {
+		t.Errorf("Expected status 200 or 301, got %d", w.Code)
+	}
+}