fix: resolve Next.js docker build endpoints and handle async map icons

frontend/Dockerfile

@@ -11,6 +11,10 @@ WORKDIR /app
 COPY --from=deps /app/node_modules ./node_modules
 COPY . .
 ENV NEXT_TELEMETRY_DISABLED 1
+# NEXT_PUBLIC_* vars must exist at build time for Next.js to inline them.
+# Default empty = auto-detect from browser hostname at runtime.
+ARG NEXT_PUBLIC_API_URL=""
+ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
 RUN npm run build
 
 FROM base AS runner