Build Next.js apps with Webpack instead of Turbopack

Next 16 defaults `next build` to Turbopack, which requires native SWC
bindings unavailable for Alpine musl from our npm mirror (only the WASM
fallback loads). Pass --webpack so the build uses the WASM SWC fallback
and succeeds inside the Docker images.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/koja/package.json

@@ -4,7 +4,7 @@
   "private": true,
   "scripts": {
     "dev": "next dev",
-    "build": "next build",
+    "build": "next build --webpack",
     "start": "next start",
     "lint": "next lint",
     "cap:sync": "cap sync android",