chore: initial project scaffold with admin web, backend, desktop client, and deployment setup

Add monorepo structure for NexaVPN WireGuard control plane including:
- .gitignore for node_modules, build artifacts, and environment files
- README with project overview, monorepo layout, and quick start guide
- Admin web UI with React, Vite, TypeScript, and nginx reverse proxy
- API client with type definitions for users, devices, policies, gateways, and audit logs
- Admin pages for dashboard, users, devices, policies, g

backend/go.mod

@@ -0,0 +1,11 @@
+module github.com/nexavpn/nexavpn/backend
+
+go 1.23.0
+
+require (
+	github.com/go-chi/chi/v5 v5.2.1
+	github.com/golang-jwt/jwt/v5 v5.2.1
+	github.com/google/uuid v1.6.0
+	github.com/jackc/pgx/v5 v5.7.2
+	golang.org/x/crypto v0.36.0
+)