Support running same user on multi machines (Devices/Agents relation management)

**Motivation**
I am trying to solve the common case of having both a workstation and laptop where we need to use the desktop app.

Currently you can use both but they are like intefering, agents duplicating, and you can't chose on which machine should the agent run.

**Proposed solution**

Devices management area with eventual features or agents feature switches.

Prevent creating new agents on new machine onboarding.

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Can't effectively search issues, think that triage loop will do it better.

No files, tests, or entry points are named. Start by mapping the desktop app's machine onboarding and agent/device management, then clarify the expected multi-machine behavior: avoid duplicate agents, prevent unwanted agent creation, and allow choosing which machine runs an agent.

Written by the indexing model from the issue text.