Multi-Session per Workspace (launcher and UI, session quotas, documentation)

[achtsnits]

Enables workspace members of a team to run multiple concurrent Datalab sessions, each connected to its own isolated (namespace- or vCluster-based) compute environment.

Goal:
Supports parallel work by multiple users, each running interactive sessions simultaneously while remaining connected to the shared buckets and storage of their workspace team.

The planned enhancement introduces

• a configurable maximum number of concurrent sessions in addition to the default one, building on the existing capabilities to
• maintain isolated compute environments with
• shared storage continuity and per-session policy controls.