Fix NODE_OPTIONS heap size for Vite build (896MB → 6144MB)

OOM observed during rendering-chunks phase at 896MB and 3072MB.
6144MB heap with 8GB LXC memory is confirmed working.
Also update README rebuild command to match.

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

cinny/lotus-build.sh

@@ -95,7 +95,7 @@ echo "Running npm ci..."
 npm ci 2>&1 | tail -5
 
 rm -rf "$BUILD_DIR"
-export NODE_OPTIONS='--max_old_space_size=896'
+export NODE_OPTIONS='--max_old_space_size=6144'
 echo "Building $LATEST_TAG..."
 npm run build 2>&1 | tail -10