fix: frontend Dockerfile npm ci → npm install (kein lockfile vorhanden)

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

frontend/Dockerfile

@@ -1,8 +1,8 @@
 FROM node:20-alpine AS builder
 
 WORKDIR /app
-COPY package.json package-lock.json* ./
-RUN npm ci
+COPY package.json ./
+RUN npm install
 COPY . .
 RUN npm run build