fix: hardcode Harbor cache URLs in FROM lines to bypass Docker Hub rate limits

frontend/Dockerfile

@@ -1,7 +1,6 @@
 # Stage 1: Build
 # Base images: uses Harbor proxy cache in CI, falls back to Docker Hub externally
-ARG BASE_REGISTRY=docker.io
-FROM ${BASE_REGISTRY}/library/node:24-alpine AS build
+FROM registry.celestium.life/dockerhub-cache/library/node:24-alpine AS build
 WORKDIR /app
 COPY package.json package-lock.json ./
 RUN npm ci
@@ -12,8 +11,7 @@ ARG VITE_RISK_ENGINE_URL=""
 RUN npm run build
 
 # Stage 2: Serve (unprivileged nginx, runs as uid 101, port 8080)
-ARG BASE_REGISTRY=docker.io
-FROM ${BASE_REGISTRY}/nginxinc/nginx-unprivileged:alpine
+FROM registry.celestium.life/dockerhub-cache/nginxinc/nginx-unprivileged:alpine
 COPY --from=build /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf
 EXPOSE 8080