Add registration bootstrap flow and lobby registration configuration

[Copilot]

Summary

Adds a registration flow parallel to the existing invite flow and lets Lobby expose a dedicated registration destination. The change includes the AuthProxy runtime path, configuration surface, Aspire helpers, and reorganized lobby docs for wiring the flow end to end.

authproxy.WithLobbyRegistration(lobbyResource, "/register");
// Sets Cratis:AuthProxy:Invite:Lobby:Registration:BaseUrl

Added

• Added a /register bootstrap flow that starts authentication and carries registration state across the login round-trip (Registration support #49)
• Added Lobby.Registration.BaseUrl configuration for redirecting authenticated users into the lobby registration experience (Registration support #49)
• Added Aspire helpers for configuring the lobby registration URL from either a raw URL or an Aspire resource reference (Registration support #49)
• Added focused specs covering registration bootstrap, post-auth redirect, and middleware bypass behavior (Registration support #49)

Changed

• Updated ingress middleware to treat registration like invite bootstrap so /register is not intercepted by tenant-selection or no-tenant lobby redirects (Registration support #49)
• Extended the shared Service configuration model with a Registration endpoint alongside existing frontend/backend endpoints (Registration support #49)
• Reorganized onboarding documentation into a dedicated lobby section with separate pages for invitation for creating organization, invitation to organization, and registration (Registration support #49)

Fixed

• Fixed the missing path for starting a registration flow without an invitation (Registration support #49)
• Fixed Lobby configuration so it can direct users to a dedicated registration URL instead of only the generic frontend (Registration support #49)

Removed

• No removals in this PR (Registration support #49)

Security

• Preserved the existing short-lived, HTTP-only cookie pattern for auth bootstrap state in the new registration flow (Registration support #49)

Deprecated

• No deprecations in this PR (Registration support #49)

[einari]

@copilot Lets organize the documentation differently - create a folder called lobby and separate out the different invitation flows and registration into separate pages, keeping at least 3 pages. Invitation for creating org, invitation to org and registration. Registration also means registering an organization - otherwise you should be invited, just a note. :)

[Copilot]

@copilot Lets organize the documentation differently - create a folder called lobby and separate out the different invitation flows and registration into separate pages, keeping at least 3 pages. Invi...

Addressed in 80430b7. The docs are now organized under Documentation/configuration/lobby/ with separate pages for invitation for creating organization, invitation to organization, and registration, and the registration page now calls out that it is for creating an organization rather than joining an existing one.