Proof of concept: Studio 2.0 combined UI

[shaunandrews]

This intentionally combines the two new UI experiments: Agentic UI and Desks. The result is useful as a prototype, but it does not fully resolve many side effects.

The awkward part is that Agentic UI and Desks each currently have their own ideas about conversations and site selection. We may want to reduce Desks into a more focused visual layer, perhaps renamed Canvas, and let the Agentic UI own conversations, session history, and app navigation. Another direction to explore is whether each site in the sidebar should have its own Desk/Canvas.

Testing Instructions

• Set ENABLE_DESKS_UI_SWITCH=true.
• Open default Studio Settings and verify it shows Switch to Studio 2.0, not separate Agentic UI and Desks UI choices.
• Switch to Studio 2.0 and verify the Agentic shell opens.
• Confirm the sidebar includes Desk.
• Open Desk and verify the global Desk is embedded in the right pane while the Agentic sidebar remains visible.
• From the Desk menu, switch between sites and verify the app stays in the Studio 2.0 shell.
• From Studio 2.0 Settings, switch back to Default Studio UI while logged out.