frontend: build with webpack so Docker images build on arm64

Next 16's default Turbopack build has no native bindings for linux/arm64 in
the Alpine image, so `next build` failed in Docker ("Turbopack is not
supported on this platform"). Switch the production build to webpack, which
Next recommends for this case and which builds on every arch.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "scripts": {
     "dev": "next dev",
-    "build": "next build",
+    "build": "next build --webpack",
     "start": "next start",
     "lint": "eslint",
     "check-locales": "node scripts/check-locales.mjs"