Addresses issue in https://github.com/dyad-sh/dyad/issues/1849

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fixes Pro users seeing an “Upgrade to Dyad Pro” prompt on provider
rate-limit errors and cleans up noisy fallback details in error
messages. Pro users now get the correct message, and errors are shorter
and clearer.

- **Bug Fixes**
- Show the upgrade link only when not on Pro for rate-limit errors
(Gemini and generic provider errors).
- Trim verbose fallback lists by splitting on "Fallbacks=[{" to keep the
fallback model’s error while removing the rest.

<sup>Written for commit e0f69dcda4b4aa0f1be2af3b1af163d84e995fe7.
Summary will update automatically on new commits.</sup>

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Avoids Pro upsell for Pro users on rate-limit errors and trims
fallback details more precisely in error messages.
> 
> - **Chat error handling (`src/components/chat/ChatErrorBox.tsx`)**:
> - **Rate-limit messaging**: Show Dyad Pro upgrade link only when
`!isDyadProEnabled` (prevents upsell for existing Pro users).
> - **Fallback error trimming**: Truncate error text at `"Fallbacks=[{"`
instead of `"Fallbacks="` to retain fallback model error context.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
e0f69dcda4b4aa0f1be2af3b1af163d84e995fe7. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->