Add error handling to diagnose cp failure

Show directory contents if cp fails.

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

Dockerfile

@@ -2,7 +2,7 @@ FROM node:20-slim
 
 ENV DEBIAN_FRONTEND=noninteractive
 
-# Build timestamp: 2026-01-13T00:50
+# Build timestamp: 2026-01-13T00:55
 
 # Install git and git-lfs
 RUN apt-get update && apt-get install -y git git-lfs && rm -rf /var/lib/apt/lists/*
@@ -32,7 +32,12 @@ RUN esbuild backend/src/server.ts --bundle --platform=node --target=node20 --for
     --external:express --external:socket.io --external:cors --external:dotenv --external:uuid
 
 # Copy frontend dist (with LFS files properly resolved)
-RUN mkdir -p app && cp -r /tmp/repo/trigo-web/app/dist ./app/
+RUN mkdir -p app && \
+    (cp -r /tmp/repo/trigo-web/app/dist ./app/ || \
+    (echo "=== CP FAILED ===" && \
+     echo "Source exists:" && ls -la /tmp/repo/trigo-web/app/ && \
+     echo "Source dist:" && ls -la /tmp/repo/trigo-web/app/dist/ 2>&1 || echo "dist not found" && \
+     exit 1))
 
 # Copy public and other files
 RUN cp -r /tmp/repo/trigo-web/public ./