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

Makefile

@@ -0,0 +1,41 @@
+.PHONY: all build test clean backend frontend run dev
+
+all: build
+
+build: frontend-build backend-build
+
+backend-build:
+	cd backend && go build -o ../bin/markdown-editor-backend
+
+frontend-build:
+	cd frontend && npm run build
+	@mkdir -p backend/static
+	@cp -r frontend/dist/* backend/static/
+	@ln -sf backend/static static
+
+test: backend-test frontend-test
+
+backend-test:
+	cd backend && go test ./... -v
+
+frontend-test:
+	cd frontend && npm test
+
+dev:
+	@echo "Starting development servers..."
+	@echo "Backend will run on http://127.0.0.1:8080"
+	@make -j2 dev-backend dev-frontend
+
+dev-backend:
+	cd backend && go run main.go --data-dir=./data
+
+dev-frontend:
+	cd frontend && npm run dev
+
+run: build
+	./bin/markdown-editor-backend --data-dir=./data
+
+clean:
+	rm -rf bin/
+	rm -rf backend/static/
+	rm -rf frontend/dist/