Fix UI Docker build: use native platform for npm ci

The CI runner is ARM64 and buildx was running npm ci under QEMU
amd64 emulation, causing a V8 crash. Use --platform=$BUILDPLATFORM
on the build stage so Node runs natively, matching the server Dockerfile.

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

ui/Dockerfile

@@ -1,4 +1,4 @@
-FROM node:22-alpine AS build
+FROM --platform=$BUILDPLATFORM node:22-alpine AS build
 WORKDIR /app
 
 COPY package.json package-lock.json ./