Upgrade Docker frontend stage from Node 24 to Node 25

Aligns the Docker build environment with the local development setup
which already uses Node 25.

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

Dockerfile

@@ -1,5 +1,5 @@
 # Stage 1: Build frontend
-FROM node:24-alpine AS frontend-build
+FROM node:25-alpine AS frontend-build
 WORKDIR /app/frontend
 COPY frontend/package.json frontend/package-lock.json ./
 RUN npm ci