Implement new navigation/sidebar

[bracesproul]

Implement the new sidebar/navigation for the authenticated view.

• new agent -> open the create agent dialog
• agent library -> /agents page
• settings -> /settings

workspace:

• dropdown for rendering agents (can pull the one we show when you visit the chat page rn). should default to the default agent
  • we should track this selection in query params agentId. this means on first page load, we should fetch the assistants from the useAgentsContext hook, and update the query params to be the default agent's ID.
  • we'll need a loading state for this too.
• chat -> /chat
• inbox -> /inbox