Fix Docker build: upgrade npm to 11 to match lockfile format

package-lock.json was generated with npm 11; node:20-alpine ships with
npm 10 which can't resolve optional esbuild platform packages from the
newer lockfile format.

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

Dockerfile

@@ -2,7 +2,7 @@
 FROM node:20-alpine AS builder
 
 WORKDIR /app
-RUN apk add --no-cache python3 make g++
+RUN apk add --no-cache python3 make g++ && npm install -g npm@11
 COPY package*.json ./
 RUN npm ci
 
@@ -13,7 +13,7 @@ RUN npm run build
 FROM node:20-alpine
 
 WORKDIR /app
-RUN apk add --no-cache python3 make g++
+RUN apk add --no-cache python3 make g++ && npm install -g npm@11
 
 # Copy package files and install production deps only
 COPY package*.json ./