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/internal/apiutil/respond.go

@@ -0,0 +1,26 @@
+package apiutil
+
+import (
+	"encoding/json"
+	"net/http"
+)
+
+type ErrorResponse struct {
+	Error struct {
+		Code    string `json:"code"`
+		Message string `json:"message"`
+	} `json:"error"`
+}
+
+func JSON(w http.ResponseWriter, status int, payload any) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	_ = json.NewEncoder(w).Encode(payload)
+}
+
+func Error(w http.ResponseWriter, status int, code, message string) {
+	resp := ErrorResponse{}
+	resp.Error.Code = code
+	resp.Error.Message = message
+	JSON(w, status, resp)
+}