fix: Ensure public directory exists in Docker build

- Add mkdir -p public in builder stage
- Add public/.gitkeep placeholder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/Dockerfile

@@ -19,6 +19,9 @@ ARG NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY
 ENV NEXT_PUBLIC_API_URL=$NEXT_PUBLIC_API_URL
 ENV NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=$NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY
 
+# Ensure public directory exists
+RUN mkdir -p public
+
 RUN npm run build
 
 # Stage 3: Runner