Design error screens and failure states for the chat app

[smeragoel]

Summary

The chat app currently has no designed error or failure states in Figma. We need Figma designs for the various ways chat can fail or degrade, built on the new Nebari design system components (consistent with #115).

Error states to design

• Insufficient permissions
• Storage not enabled

Scope

• Design each error state in Figma using new design system components.
• Define copy/tone for error messages (clear, actionable, no raw stack traces).
• Specify recovery actions (retry, refresh, re-auth) for each state.
• Flag any error patterns not yet covered by the design system as candidates for new shared components.

Out of scope

• Implementing the error states in code.