feat: register the desktop app as an authenticated worker

[ibourgeois]

Context

As a connected Katra instance, I need desktop clients to register as authenticated workers so they can contribute local compute resources while still acting as normal clients for the active instance.

Acceptance Criteria

• A desktop app can register itself as a worker for the currently active authenticated instance.
• Desktop clients continue acting as normal clients while also being eligible workers.
• Worker participation is enabled by default for desktop clients, but can be disabled in client settings or by the connected server instance.
• Only authenticated instances can use a desktop app as a worker.

Notes

The desktop app should always be a client first. Worker registration is an additional capability layered onto the active authenticated instance connection, not a separate anonymous worker mode.

Out of Scope

• Queue routing policy between default queues and remote-worker queues.
• Detailed data privacy / cleanup guarantees during job execution.