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/server/server_test.go

@@ -0,0 +1,36 @@
+package server
+
+import (
+	"context"
+	"testing"
+)
+
+func TestServer_Shutdown(t *testing.T) {
+	s := NewServer("./testdata", 8889, "127.0.0.1")
+
+	ctx := context.Background()
+	err := s.Shutdown(ctx)
+	if err != nil {
+		t.Errorf("Shutdown error: %v", err)
+	}
+}
+
+func TestNewServer(t *testing.T) {
+	s := NewServer("./testdata", 8890, "127.0.0.1")
+	
+	if s == nil {
+		t.Error("Server is nil")
+	}
+	
+	if s.dataDir != "./testdata" {
+		t.Errorf("Expected dataDir './testdata', got '%s'", s.dataDir)
+	}
+	
+	if s.port != 8890 {
+		t.Errorf("Expected port 8890, got %d", s.port)
+	}
+	
+	if s.host != "127.0.0.1" {
+		t.Errorf("Expected host '127.0.0.1', got '%s'", s.host)
+	}
+}