feat(markdown-editor): implement wysiswyg markdown editor with live preview

- Build Go backend with Cobra CLI and REST API
  - CRUD operations for markdown files (GET, POST, PUT, DELETE)
  - File storage with flat .md file structure
  - Comprehensive logrus logging with JSON format
  - Static asset serving for frontend

- Build React/TypeScript frontend with Tailwind CSS
  - Markdown editor with live GFM preview
  - File management UI (list, create, open, delete)
  - Theme system (Dark/Light/System) with persistence
  - Responsive design (320px mobile, 1920px desktop)

- Add comprehensive test coverage
  - Backend: API, storage, and logger tests (13 tests passing)
  - Frontend: Editor and App component tests

- Setup Nix development environment with Go, Node.js, and TypeScript

backend/go.mod

@@ -0,0 +1,14 @@
+module github.com/markdown-editor
+
+go 1.25.5
+
+require (
+	github.com/sirupsen/logrus v1.9.4
+	github.com/spf13/cobra v1.10.2
+)
+
+require (
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/spf13/pflag v1.0.9 // indirect
+	golang.org/x/sys v0.13.0 // indirect
+)