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

backend/src/server.ts

@@ -2,5 +2,5 @@ import { buildApp } from "./app.js";
 import { loadConfig } from "./config.js";
 const config = loadConfig(process.env);
 const app = buildApp(config);
-app.listen({ port: config.port, host: "127.0.0.1" })
+app.listen({ port: config.port, host: config.host })
   .then((addr) => console.log(`backend listening on ${addr}`));