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/App.vue

@@ -0,0 +1,30 @@
+<script setup lang="ts">
+import HelloWorld from './components/HelloWorld.vue'
+</script>
+
+<template>
+  <div>
+    <a href="https://vite.dev" target="_blank">
+      <img src="/vite.svg" class="logo" alt="Vite logo" />
+    </a>
+    <a href="https://vuejs.org/" target="_blank">
+      <img src="./assets/vue.svg" class="logo vue" alt="Vue logo" />
+    </a>
+  </div>
+  <HelloWorld msg="Vite + Vue" />
+</template>
+
+<style scoped>
+.logo {
+  height: 6em;
+  padding: 1.5em;
+  will-change: filter;
+  transition: filter 300ms;
+}
+.logo:hover {
+  filter: drop-shadow(0 0 2em #646cffaa);
+}
+.logo.vue:hover {
+  filter: drop-shadow(0 0 2em #42b883aa);
+}
+</style>