fix(error-page): render gracefully when panel prop is missing

[maherelgamil]

Summary

The error page (resources/js/pages/error.tsx) assumed panel was always provided. When a 500 is thrown outside the panel middleware stack — or before RenderMonorailErrorPages has had a chance to inject panel data — the page receives panel === undefined and crashes at:

```ts
const __ = create__(panel.translations ?? {});
// ^^^^^ TypeError: Cannot read properties of undefined
```

The React render error then masks the actual server error, making the underlying 500 hard to diagnose.

Fix

• Make the panel prop optional.
• Use panel?.translations and panel?.path with sane fallbacks.
• When panel is missing, skip PanelShell (which strictly requires it) and render a centered standalone layout so the status, title, and message still reach the user.

Reproduction

1. Hit any route that 500s outside the panel scope (e.g., a missing controller for /opportunities).
2. Inertia renders the Monorail error page without a panel prop.
3. Without this fix: white screen + console TypeError: Cannot read properties of undefined (reading 'translations').
4. With this fix: standalone error card with status code, title, message, and a link to /.

Test plan

• Trigger a 500 outside /admin and confirm the error page renders
• Trigger a 404 inside /admin and confirm panel shell still wraps the error (no regression)