docs: fix dashboard backend source paths

[haosenwang1018]

Summary

• update the root README source-run instructions to use packages/openmemory-js instead of the removed backend directory
• update dashboard/CHAT_SETUP.md to point the dashboard setup guide at the same real backend path

Why

Issue #133 reports that the dashboard exists in the repo but is not actually consumable in non-Docker installs.

One concrete source of confusion is that the repository docs still instruct users to run the backend from:

cd backend

But the current repository layout uses:

cd packages/openmemory-js

Fixing these paths removes a guaranteed setup failure in the source-based dashboard flow.

Testing

• verified the repo no longer contains cd backend in the updated source-run instructions
• confirmed both docs now point to packages/openmemory-js

Related to #133