🚚(frontend) delete useless nesting

Update frontend to be the root folder of the frontend sources,
instead of nesting them in a folder named as the application.

We only work on a single frontend app as of today, nesting
sources doesn't add any value, even though the initial organization
was more extensible.

src/frontend/vite.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vite'
+import react from '@vitejs/plugin-react'
+
+// https://vitejs.dev/config/
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    port: 3000,
+  },
+})