Add custom error screen

[stylessh]

Summary

• Add a custom error screen component that matches the dashboard design system (centered layout, destructive icon, retry button)
• Wire it into both __root and _protected routes as errorComponent
• Replace the default unstyled TanStack error boundary

Test plan

• Trigger an error (e.g. throw in a loader) and verify the custom screen renders
• Verify "Try again" button resets the error and re-fetches
• Check dark mode appearance