Add error boundaries and loading states

[superposition]

Description

Implement graceful error handling and loading UI.

Tasks

• Create ErrorBoundary component
• Add fallback UI for crashed components
• Implement skeleton loaders for panels
• Add loading spinner for surface
• Show error toast for API failures
• Add retry button for failed requests
• Log errors to console with context
• Handle WebGL context loss gracefully

Acceptance Criteria

• App doesn't crash on errors
• Loading states are clear
• Users can recover from failures

Labels

web-app, frontend, ux