feat: implement WYSIWYG markdown editor with Go backend and React frontend

- Backend: Go HTTP server with Cobra CLI (--data-dir, --port, --host flags)
- CRUD REST API for markdown files with JSON error responses
- File storage in flat directory structure (flat structure, .md files only)
- Comprehensive logrus logging for all operations
- Static file serving for frontend build (./frontend/dist)
- Frontend: React + TypeScript + Tailwind CSS
- Markdown editor with live GFM preview
- File management: list, create, open, save, delete
- Theme system (Dark, Light, System) with persistence
- Responsive design for desktop and mobile
- Backend tests (storage, API handlers) and frontend tests

Makefile

@@ -0,0 +1,26 @@
+.PHONY: all build test frontend backend frontend-build backend-test
+
+all: build
+
+build: backend frontend-build
+	@echo "Build complete"
+
+test: backend-test frontend-test
+
+backend:
+	go build -o eval ./cmd
+
+backend-test:
+	go test ./internal/storage ./internal/api -v
+
+frontend:
+	npm --prefix frontend install
+
+frontend-build:
+	npm --prefix frontend run build
+
+frontend-test:
+	npm --prefix frontend run test
+
+frontend-dev:
+	npm --prefix frontend run dev