Add package-lock.json to gitignore

Project uses Yarn, so npm's package-lock.json should be ignored.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: justin808

.gitignore

@@ -48,6 +48,9 @@ client/app/libs/i18n/default.js
 yarn-debug.log*
 .yarn-integrity
 
+# npm lock file (project uses yarn)
+package-lock.json
+
 lib/bs
 /lib/ocaml