Fix: capture full Vite build output for diagnostics, add --logLevel info

Dockerfile

@@ -25,9 +25,12 @@ RUN npm install --legacy-peer-deps 2>&1 && \
 COPY frontend/ ./
 
 RUN echo "=== starting vite build ===" && \
-  NODE_ENV=production ./node_modules/.bin/vite build 2>&1 && \
-  echo "=== dist ===" && \
-  ls -la dist/
+  sh -c 'NODE_ENV=production ./node_modules/.bin/vite build --logLevel info 2>&1; \
+    CODE=$?; \
+    echo "=== Vite exit code: $CODE ==="; \
+    exit $CODE'
+
+RUN echo "=== dist ===" && ls -la dist/
 
 # ============================================
 # Stage 2: Python Backend + Serve Frontend