fix: remove npm self-update in Docker build; use npm ci

npm install -g npm@latest corrupts npm's own module tree inside Alpine,
breaking subsequent installs. Use npm ci instead (faster, deterministic).

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

Dockerfile

@@ -2,7 +2,7 @@
 FROM node:22-alpine AS build-frontend
 WORKDIR /build
 COPY frontend/package.json frontend/package-lock.json* ./
-RUN npm install -g npm@latest --quiet && npm install
+RUN npm ci --quiet
 COPY frontend/ .
 RUN npm run build