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)

vite.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vite'
+import vue from '@vitejs/plugin-vue'
+
+// https://vite.dev/config/
+export default defineConfig({
+  plugins: [vue()],
+})