Stack integration: Stack login and fyra.zone subdomains

[halfcyan]

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

if it's through stack dashboard, they can log in with their stack credentials
offer subdomains for users who use a stack vps for their dashboard

Goal

The dashboard is hosted centrally by default and uses Stack authentication there. Self-hosting stays fully supported bc Stack integration is additive, not a replacement. Users on Stack get a *.fyra.zone subdomain out of the box. Part spike, part implementation: scope depends on what Stack exposes.

Tasks

• Investigate Stack auth surface; document the integration shape
• Login with Stack credentials when centrally hosted (default)
• Self-hosted auth path keeps working (local hosting mode)
• fyra.zone subdomain provisioning for Stack-hosted dashboards
• Apps on the host can use Stack credentials where appropriate
• Tests

Related: #7 (Integrated SSO — app-level use of Stack credentials), Ultramarine-Linux/server-oobe#3 (OOBE fyra-dash authorization step consumes this).