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
• a configurable overall storage limit for the team
  building on the existing capabilities to maintain isolated compute environments with shared storage continuity and per-session policy controls.

Also add corresponding UI elements, i.e.

• list all together with session state in workspace UI.

In addition this issue tracks the topic to protect access to a running session (see https://eoepca.slack.com/archives/C07KHPXA1AP/p1776768057351689) so we have a good "default" setup