PR #778: feat(auth): add multi-workspace tenant login

Release Notes

Summary

Users who belong to more than one Stellarbridge workspace can now sign in through a central login page, choose which workspace to enter, switch between workspaces from the dashboard, and sign out of one workspace without ending sessions in others.

Changes

Added a central workspace login flow that authenticates users and routes them to the correct workspace after sign-in.
Added workspace selection and switching from the account menu in the dashboard.
Signing out now ends only the current workspace session; other workspace sessions and the identity-provider single sign-on session remain active.
After signing out, users can sign back into the same workspace or choose a different workspace.
Strengthened sign-in validation, including organization binding, verified-email checks, and tenant-local access controls.
Tenant deployments continue to use the default login URL (https://login.stellarbridge.app)

Impact

Behavior change: Sign-in, workspace switching, and sign-out now operate per workspace rather than as a single global session. Users with access to multiple workspaces will see workspace selection during login and can move between workspaces without signing out everywhere.

Classification

Auth, UI, Tenant Workflow, Security