Wire local system steps to the host: hostname, admin user, network

[halfcyan]

Part of the Ultramarine Server roadmap (Outline: Server):

allow users to create apps and run OOBE and stuff where they do not have to touch a terminal

Goal

The step skeleton exists (welcome → devicename → whoareyou → password → internet → tweaks → tetra → fyra-dash → complete) with fixture state; only network/operations API routes exist. This issue wires the local-system steps to the real host via tetra.

Tasks

• Device name step applies through tetra settings.set_hostname
• Administrator step creates the account via tetra users.create + users.set_password
• Internet step backed by live data from tetra network.interfaces; config changes via network.set_config
• Keyboard layout + tweaks persisted
• Operation status/retry UX driven by /api/oobe/operations
• Tests