feat: unify on single repo-root compose file for dev and prod

frontend/vite.config.ts

@@ -1,13 +1,15 @@
 import { defineConfig } from "vite";
 import react from "@vitejs/plugin-react";
 
+// Both dev server and preview proxy /api to the backend service.
+// `api` is the compose service name, identical in dev and prod.
 export default defineConfig({
   plugins: [react()],
   server: {
     port: 3000,
     proxy: {
       "/api": {
-        target: "http://localhost:8000",
+        target: "http://api:8000",
         changeOrigin: true,
       },
     },
@@ -16,8 +18,7 @@ export default defineConfig({
     port: 3000,
     proxy: {
       "/api": {
-        // Overridable at container start (prod compose: http://app:8000).
-        target: process.env.VITE_API_PROXY_TARGET ?? "http://localhost:8000",
+        target: "http://api:8000",
         changeOrigin: true,
       },
     },