feat: implement WYSIWYG markdown editor with Go backend and React frontend

Implements full markdown editor application with:

Backend (Go):
- Cobra CLI with --data-dir, --port, --host flags
- REST API for CRUD operations on markdown files
- File storage on disk with flat structure
- Logrus logging for all operations
- Static asset serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend (React + TypeScript + Tailwind):
- Markdown editor with live GFM preview
- File management UI (list, create, open, save, delete)
- Theme system (Dark, Light, System) with persistence
- Responsive design (320px to 1920px)
- Component tests for core functionality

Integration:
- Full CRUD workflow from frontend to backend
- Static asset serving verified
- All tests passing (backend: 2/2, frontend: 6/6)

Files added:
- Backend: API handler, logger, server, tests
- Frontend: Components, tests, config files
- Build artifacts: compiled backend binary and frontend dist
- Documentation: README and implementation summary

README.md

@@ -0,0 +1,126 @@
+# WYSIWYG Markdown Editor
+
+A markdown editor with live preview, file management, and theme switching.
+
+## Features
+
+- **Markdown Editor**: Write markdown with live GitHub Flavored Markdown preview
+- **File Management**: Create, open, save, and delete markdown files
+- **Theme System**: Dark, Light, and System themes
+- **Responsive Design**: Works on desktop and mobile devices
+
+## Running the Application
+
+### Prerequisites
+
+- Node.js (v18+)
+- Go (v1.21+)
+- npm or yarn
+
+### Backend
+
+```bash
+# Build the backend
+cd backend
+make build
+
+# Run the backend
+./bin/markdown-editor
+
+# Or with custom flags
+./bin/markdown-editor --data-dir ./my-data --port 3000 --host 0.0.0.0
+```
+
+### Frontend
+
+```bash
+# Install dependencies
+cd frontend
+npm install
+
+# Build the frontend
+npm run build
+
+# Run in development mode
+npm run dev
+```
+
+### Running Both
+
+1. Build the frontend:
+   ```bash
+   cd frontend
+   npm run build
+   ```
+
+2. Run the backend (it will serve the built frontend):
+   ```bash
+   cd backend
+   ./bin/markdown-editor
+   ```
+
+3. Open your browser to `http://localhost:8080`
+
+## Development
+
+### Running Tests
+
+```bash
+# Backend tests
+cd backend
+make test
+
+# Frontend tests
+cd frontend
+npm test
+```
+
+### Project Structure
+
+```
+backend/
+  cmd/
+    backend/
+      main.go
+  internal/
+    api/
+      api.go
+    logger/
+      logger.go
+    server/
+      server.go
+  tests/
+    api_test.go
+  go.mod
+  go.sum
+  Makefile
+
+frontend/
+  src/
+    App.tsx
+    main.tsx
+    index.css
+    setupTests.ts
+    App.test.tsx
+  package.json
+  vite.config.ts
+  tailwind.config.js
+  postcss.config.js
+  tsconfig.json
+  index.html
+
+Makefile
+README.md
+SPEC.md
+```
+
+## API Endpoints
+
+- `GET /api/{filename}.md` - Get markdown file content
+- `POST /api/{filename}.md` - Create a new markdown file
+- `PUT /api/{filename}.md` - Update an existing markdown file
+- `DELETE /api/{filename}.md` - Delete a markdown file
+
+## License
+
+MIT