feat(deploy): configurable backend HOST + env-driven vite base/assetsDir

- backend: HOST env (default 127.0.0.1, dev-safe; 0.0.0.0 in container)
- frontend: VITE_BASE drives base/assetsDir/manifest for portal embedding
- backward compatible: no env => identical to previous behavior

frontend/vite.config.ts

@@ -2,11 +2,15 @@ import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
 import path from "path";
 
-export default defineConfig({
-  plugins: [react()],
-  resolve: {
-    alias: {
-      "@": path.resolve(__dirname, "./src"),
-    },
-  },
+export default defineConfig(() => {
+  const embedBase = process.env.VITE_BASE; // "/datamart-builder/assets/" in embedded; undefined = standalone
+  return {
+    plugins: [react()],
+    // base: prefisso pubblico degli asset. Default "/" (standalone).
+    // assetsDir vuoto in embedded → asset alla root di dist/ così il proxy
+    // /datamart-builder/assets/ → frontend-root mappa 1:1 (niente /assets/assets/).
+    base: embedBase ?? "/",
+    build: { manifest: true, outDir: "dist", assetsDir: embedBase ? "" : "assets" },
+    resolve: { alias: { "@": path.resolve(__dirname, "./src") } },
+  };
 });