refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

src/main.ts

@@ -0,0 +1,5 @@
+import { createApp } from 'vue'
+import './style.css'
+import App from './App.vue'
+
+createApp(App).mount('#app')