feat: implement WYSIWYG markdown editor

Add full-stack markdown editor with Go backend and React frontend.

Backend:
- Cobra CLI with --data-dir, --port, --host flags
- REST API for markdown file CRUD operations
- File storage with flat directory structure
- logrus logging for all operations
- Static file serving for frontend
- Comprehensive tests for CRUD and static assets

Frontend:
- React + TypeScript + Vite + Tailwind CSS
- Live markdown preview with marked (GFM)
- File management: list, create, open, save, delete
- Theme system: Dark/Light/System with persistence
- Responsive design (320px to 1920px)
- Component tests for Editor, Preview, Sidebar

Build:
- Makefile for build, test, and run automation
- Single command testing (make test)

Closes SPEC.md requirements

README.md

@@ -0,0 +1,121 @@
+# WYSIWYG Markdown Editor
+
+A full-stack markdown editor with live preview, built with Go backend and React/TypeScript frontend.
+
+## Features
+
+- **Live Preview**: Real-time GFM markdown rendering as you type
+- **File Management**: Create, open, edit, save, and delete markdown files
+- **Three Themes**: Light, Dark, and System (auto-detect) with theme persistence
+- **Responsive Design**: Works on mobile (320px) and desktop (1920px)
+- **CRUD API**: RESTful API for file operations
+
+## Tech Stack
+
+- **Backend**: Go with Cobra CLI, logrus logging, standard HTTP server
+- **Frontend**: React, TypeScript, Tailwind CSS, Vite, marked (GFM parser)
+- **Storage**: Local filesystem (flat structure)
+
+## Quick Start
+
+### Prerequisites
+
+- Go 1.21+
+- Node.js 20+
+- Nix (optional, for development environment)
+
+### Development
+
+1. Enter the development environment (with Nix):
+   ```bash
+   nix develop
+   ```
+
+2. Start the backend:
+   ```bash
+   make dev-backend
+   # or: cd backend && go run main.go
+   ```
+
+3. Start the frontend (in another terminal):
+   ```bash
+   make dev-frontend
+   # or: cd frontend && npm run dev
+   ```
+
+4. Open http://localhost:5173 (frontend dev server) or http://127.0.0.1:8080 (backend)
+
+### Build & Run
+
+```bash
+# Build everything (frontend + backend)
+make build
+
+# Run the built application
+make run
+# or: ./bin/markdown-editor-backend
+
+# Run tests
+make test
+```
+
+### CLI Options
+
+```
+./bin/markdown-editor-backend --help
+
+Flags:
+  --data-dir string   Storage path for markdown files (default "./data")
+  --host string       Bind address (default "127.0.0.1")
+  --port int          Server port (default 8080)
+```
+
+## API Endpoints
+
+| Method | Endpoint | Description |
+|--------|----------|-------------|
+| GET | `/api/health` | Health check |
+| GET | `/api/files` | List all files |
+| POST | `/api/files` | Create new file |
+| GET | `/api/files/{name}` | Get file content |
+| PUT | `/api/files/{name}` | Update file |
+| DELETE | `/api/files/{name}` | Delete file |
+
+## Project Structure
+
+```
+.
+├── backend/
+│   ├── main.go              # CLI entry point
+│   ├── server/              # HTTP server
+│   ├── handlers/            # API handlers
+│   ├── storage/             # File storage
+│   └── static/              # Frontend build output
+├── frontend/
+│   ├── src/
+│   │   ├── components/      # React components
+│   │   ├── api/             # API client
+│   │   └── types.ts         # TypeScript types
+│   └── dist/                # Build output
+├── data/                    # Markdown files storage
+├── bin/                     # Compiled binaries
+├── Makefile                 # Build automation
+└── flake.nix                # Nix development environment
+```
+
+## Testing
+
+```bash
+# Run all tests
+make test
+
+# Backend tests only
+make backend-test
+
+# Frontend tests only
+make frontend-test
+```
+
+## License
+
+MIT