fire-planner: copy frontend/.npmrc before npm ci in Dockerfile

Without this, the legacy-peer-deps flag from .npmrc isn't visible at
npm-ci time (the file isn't part of the COPY layer that triggers the
install). CI was failing on the Visx 3.x peer-dep cap against React
19.

Verified locally with 'docker build --target frontend-builder'.

Dockerfile

@@ -3,7 +3,7 @@ FROM node:22-alpine AS frontend-builder
 
 WORKDIR /frontend
 
-COPY frontend/package.json frontend/package-lock.json ./
+COPY frontend/package.json frontend/package-lock.json frontend/.npmrc ./
 RUN npm ci
 
 COPY frontend/ ./