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

backend/Makefile

@@ -0,0 +1,17 @@
+GO=go
+
+.PHONY: all test build run clean
+
+all: build
+
+build:
+	$(GO) build -o bin/markdown-editor ./cmd/backend
+
+run:
+	$(GO) run ./cmd/backend
+
+test:
+	$(GO) test -v ./tests
+
+clean:
+	rm -rf bin