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/test/storage_test.go

@@ -0,0 +1,139 @@
+package test
+
+import (
+	"os"
+	"testing"
+
+	"github.com/evanreichard/markdown-editor/internal/logging"
+	"github.com/evanreichard/markdown-editor/internal/storage"
+)
+
+func init() {
+	logging.Init()
+}
+
+func TestStorageCRUD(t *testing.T) {
+	// Create temporary directory
+	tmpDir, err := os.MkdirTemp("", "markdown-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// Initialize storage
+	s, err := storage.New(tmpDir)
+	if err != nil {
+		t.Fatalf("Failed to create storage: %v", err)
+	}
+
+	// Test Create
+	file, err := s.Create("test.md", "# Test\n\nHello, World!")
+	if err != nil {
+		t.Fatalf("Failed to create file: %v", err)
+	}
+	if file.Name != "test.md" {
+		t.Errorf("Expected name 'test.md', got '%s'", file.Name)
+	}
+	if file.Content != "# Test\n\nHello, World!" {
+		t.Errorf("Content mismatch")
+	}
+
+	// Test Get
+	retrieved, err := s.Get("test.md")
+	if err != nil {
+		t.Fatalf("Failed to get file: %v", err)
+	}
+	if retrieved.Name != "test.md" {
+		t.Errorf("Expected name 'test.md', got '%s'", retrieved.Name)
+	}
+
+	// Test Update
+	updated, err := s.Update("test.md", "# Updated\n\nNew content")
+	if err != nil {
+		t.Fatalf("Failed to update file: %v", err)
+	}
+	if updated.Content != "# Updated\n\nNew content" {
+		t.Errorf("Update failed, content mismatch")
+	}
+
+	// Test List
+	files, err := s.List()
+	if err != nil {
+		t.Fatalf("Failed to list files: %v", err)
+	}
+	if len(files) != 1 {
+		t.Errorf("Expected 1 file, got %d", len(files))
+	}
+
+	// Test Delete
+	err = s.Delete("test.md")
+	if err != nil {
+		t.Fatalf("Failed to delete file: %v", err)
+	}
+
+	files, err = s.List()
+	if err != nil {
+		t.Fatalf("Failed to list after delete: %v", err)
+	}
+	if len(files) != 0 {
+		t.Errorf("Expected 0 files after delete, got %d", len(files))
+	}
+
+	// Test Get after delete (should fail)
+	_, err = s.Get("test.md")
+	if err != storage.ErrFileNotFound {
+		t.Errorf("Expected ErrFileNotFound, got %v", err)
+	}
+
+	t.Log("CRUD round-trip test passed")
+}
+
+func TestStorageValidation(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "markdown-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	s, err := storage.New(tmpDir)
+	if err != nil {
+		t.Fatalf("Failed to create storage: %v", err)
+	}
+
+	// Test invalid names
+	invalidNames := []string{"", "../test.md", "test.txt", "folder/test.md", "test.MD"}
+	for _, name := range invalidNames {
+		_, err := s.Create(name, "content")
+		if err != storage.ErrInvalidName {
+			t.Errorf("Expected ErrInvalidName for '%s', got %v", name, err)
+		}
+	}
+
+	t.Log("Validation test passed")
+}
+
+func TestStorageDuplicateCreate(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "markdown-test-*")
+	if err != nil {
+		t.Fatalf("Failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	s, err := storage.New(tmpDir)
+	if err != nil {
+		t.Fatalf("Failed to create storage: %v", err)
+	}
+
+	_, err = s.Create("dup.md", "content")
+	if err != nil {
+		t.Fatalf("Failed to create first file: %v", err)
+	}
+
+	// Try to create duplicate
+	_, err = s.Create("dup.md", "content 2")
+	if err == nil {
+		t.Errorf("Expected error when creating duplicate file")
+	}
+
+	t.Log("Duplicate create test passed")
+}