Remove root node_modules from repo and gitignore it

The vitest cache file under the root node_modules/ was being tracked
because .gitignore only ignored frontend/node_modules/. Untrack it and
add a root-level node_modules/ rule.

https://claude.ai/code/session_013c178p4dVRXMq7QEwXb7ZX

Author: claude

.gitignore

@@ -1,4 +1,5 @@
 # Node / Next.js
+node_modules/
 frontend/node_modules/
 frontend/.next/
 frontend/out/