Fix outdated dashboard startup command in dashboard README

[Evenss]

Description

The dashboard README currently references contextseek-dashboard, but the current CLI entrypoint appears to be contextseek desktop-server.

Related documentation:

• https://github.com/ob-labs/contextseek/blob/main/dashboard/README.md

Please update dashboard/README.md so the documented startup flow matches the actual CLI.

Acceptance criteria:

• Replace outdated contextseek-dashboard references with the correct command.
• Ensure manual build/run instructions are accurate.
• Keep the explanation consistent with the current same-origin API + SPA setup.

Documentation links

• https://github.com/ob-labs/contextseek/blob/main/dashboard/README.md

Additional context

No response