Web demo agent hangs when auth token expires

## Problem
When the authentication token expires in the web demo agent, the interface gets stuck displaying 'Assistant is thinking' without any error message or recovery mechanism.

## Expected Behavior
- Clear error message when token expires
- Prompt to re-authenticate or refresh token
- Graceful handling of expired tokens

## Current Behavior
- Agent hangs indefinitely at 'Assistant is thinking'
- No error message displayed to user
- No way to recover without refreshing the page

## Steps to Reproduce
1. Use web demo agent with auth token
2. Wait for token to expire (or manually expire it)
3. Try to perform an action
4. Observe agent hanging at 'Assistant is thinking'

## Suggested Fix
- Implement token expiration detection
- Add error handling for 401/403 responses
- Display clear error message to user
- Provide option to re-authenticate

## Impact
User experience issue - users may not understand why the agent stopped working and lose their session context when forced to refresh.