web: scaffold Vue 3 + Vite app (create-vue, lean options)

Verbatim output of 'npm create vue@latest web -- --ts --vitest
--eslint --prettier' so this commit shows exactly what the official
scaffold generates; our own changes come separately. Lean per the
agreed choices: no Router/Pinia until a second page or shared state
exists. node_modules is ignored via the scaffold's own web/.gitignore.

web/src/main.ts

@@ -0,0 +1,6 @@
+import './assets/main.css'
+
+import { createApp } from 'vue'
+import App from './App.vue'
+
+createApp(App).mount('#app')