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

README.md

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+# Markdown Editor
+
+A WYSIWYG Markdown Editor with live preview, built with Go backend and React/TypeScript frontend.
+
+## Features
+
+- **Markdown Editor** with live GitHub Flavored Markdown preview
+- **File Management**: Create, open, save, and delete markdown files
+- **Theme System**: Dark, Light, and System themes with persistence
+- **Responsive Design**: Works on desktop and mobile
+- **REST API**: Full CRUD operations for markdown files
+
+## Project Structure
+
+```
+.
+├── backend/
+│   ├── cmd/server/
+│   │   └── main.go
+│   ├── internal/
+│   │   ├── api/
+│   │   │   ├── server.go
+│   │   │   └── server_test.go
+│   │   └── storage/
+│   │       ├── storage.go
+│   │       └── storage_test.go
+│   ├── pkg/
+│   │   └── logger/
+│   │       └── logger.go
+│   └── go.mod
+├── frontend/
+│   ├── src/
+│   │   ├── components/
+│   │   ├── hooks/
+│   │   ├── lib/
+│   │   ├── types/
+│   │   ├── App.tsx
+│   │   ├── App.test.tsx
+│   │   ├── index.css
+│   │   └── index.tsx
+│   ├── public/
+│   ├── package.json
+│   ├── tsconfig.json
+│   └── tailwind.config.js
+├── flake.nix
+├── flake.lock
+└── SPEC.md
+```
+
+## Development
+
+### Using Nix
+
+The project uses Nix for development environment. Ensure you have Nix installed.
+
+```bash
+# Start development shell
+nix-shell
+
+# Run tests
+cd backend && go test ./...
+cd ../frontend && npm test
+```
+
+### Backend
+
+```bash
+cd backend
+
+# Run server with defaults
+go run cmd/server/main.go
+
+# Run with custom settings
+go run cmd/server/main.go --data-dir ./data --port 8080 --host 127.0.0.1
+
+# Run tests
+go test ./...
+```
+
+### Frontend
+
+```bash
+cd frontend
+
+# Install dependencies (in nix-shell)
+npm install
+
+# Start development server
+npm start
+
+# Build for production
+npm run build
+
+# Run tests
+npm test
+```
+
+## API Endpoints
+
+- `GET /api/files` - List all markdown files
+- `POST /api/files` - Create a new file
+- `PUT /api/files/:filename` - Update a file
+- `DELETE /api/files/:filename` - Delete a file
+- `/` - Serve frontend (SPA fallback)
+- `/static/*` - Serve static assets
+
+## Testing
+
+Run all tests:
+
+```bash
+# Backend
+cd backend && go test -v ./...
+
+# Frontend
+cd frontend && npm test
+```
+
+## Evaluation Checklist
+
+- [ ] CLI starts with defaults
+- [ ] CRUD works end-to-end
+- [ ] Static assets are properly served
+- [ ] Theme switch & persistence
+- [ ] Responsive at 320px and 1920px