Error handling and toast notifications

[GordonBeeming]

Description

Global error handling and notification system.

Acceptance Criteria

• Toast notification component (top-right corner)
• Types: success (green), error (red), warning (yellow), info (blue)
• Auto-dismiss after 5s (errors persist until dismissed)
• Connection errors show toast with "Reconnect" action
• Sidecar crash shows toast with "Restart" action
• Unhandled errors caught by global error boundary
• Error details expandable in toast