Fix npm ci failure for react-scripts 5 + React 19 peer deps

react-scripts@5.0.1 declares peer deps against React 17/18; adding
--legacy-peer-deps lets npm ci succeed without changing the lockfile.

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

Dockerfile

@@ -6,8 +6,8 @@ WORKDIR /app/client
 # Copy client package files
 COPY client/package*.json ./
 
-# Install dependencies
+# Install dependencies (--legacy-peer-deps needed for react-scripts 5 + React 19)
-RUN npm ci
+RUN npm ci --legacy-peer-deps
 
 # Copy client source
 COPY client/src ./src