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

Makefile

@@ -0,0 +1,37 @@
+.PHONY: all test backend-test frontend-test frontend-lint frontend-build clean help
+
+all: test
+
+backend-test:
+	@echo "Running backend tests..."
+	cd backend && go test -v ./test/...
+
+frontend-test:
+	@echo "Running frontend tests..."
+	cd frontend && npm run test -- --run
+
+frontend-lint:
+	@echo "Running frontend linting..."
+	cd frontend && npm run lint
+
+frontend-build:
+	@echo "Building frontend..."
+	cd frontend && npm run build
+
+test: backend-test frontend-test
+	@echo "All tests passed!"
+
+clean:
+	@echo "Cleaning..."
+	cd backend && rm -rf data
+	cd frontend && rm -rf dist node_modules
+
+help:
+	@echo "Available commands:"
+	@echo "  make test          - Run all tests"
+	@echo "  make backend-test  - Run backend tests"
+	@echo "  make frontend-test - Run frontend tests"
+	@echo "  make frontend-lint - Run frontend linting"
+	@echo "  make frontend-build - Build frontend for production"
+	@echo "  make clean         - Clean build artifacts"
+	@echo "  make help          - Show this help message"