feat: add the workspace model and active workspace switching

[ibourgeois]

Summary

Introduce workspaces as the first project-level container underneath a connection.

Why This Matters

Connections are now real, but the app still has no durable project boundary inside a connection. Workspaces are the next level of organization and will become the parent for rooms and chats.

Acceptance Criteria

• add a durable workspace model scoped to a connection
• allow a user to create at least one workspace for the active connection
• allow the shell to switch the active workspace cleanly
• active workspace state is persisted per connection
• the shell UI reflects the active workspace instead of the current placeholder state

Notes

This issue is intentionally about workspace persistence and switching only. It should not include room or chat behavior yet.