Let New project select an execution machine

[amadad]

Summary

• let New project choose among enrolled machines when more than one is available
• browse and create the local-path source on the selected host, while disabling offline machines
• preserve the existing native folder-picker flow for single-machine installs and existing add/update-source behavior
• document the machine-first project creation path

Why

The project-create contract and remote folder browser already support a host ID, but the New project UI always targeted the primary host. Multi-machine users therefore had to leave the UI and use bb project create --machine ....

This keeps the host decision in the project path dialog and passes the selected host through the existing create contract. It does not change the server, database, HTTP API, or host-daemon protocol.

Verification

• pnpm exec turbo run test --filter=@bb/app — 285 files, 2,083 tests passed
• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run lint --filter=@bb/app — 0 errors
• Prettier and git diff --check
• manual browser QA with two connected machines and one offline machine: switched machines, browsed the selected host, created a project, and confirmed its source stored the selected hostId; the offline machine remained disabled