Don't show upgrade button for non AI streaming errors (#1680)

<!-- CURSOR_SUMMARY -->
> [!NOTE]
> Show the upgrade button only for AI streaming errors by checking a
shared error prefix; introduce and use a shared constant for consistent
error messages.
> 
> - **Chat UI**
> - Update `ChatErrorBox.tsx` to display "Upgrade to Dyad Pro" only when
`error` includes `AI_STREAMING_ERROR_MESSAGE_PREFIX` and user is not Pro
(still hides on `TypeError: terminated`).
> - **IPC/Streaming**
> - In `chat_stream_handlers.ts`, prefix emitted streaming errors with
`AI_STREAMING_ERROR_MESSAGE_PREFIX` instead of hardcoded text for
consistency.
> - **Shared**
> - Add `AI_STREAMING_ERROR_MESSAGE_PREFIX` in `src/shared/texts.ts` and
import where needed.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
83b1aa3555a3c1e710ee644f7bfb64cabc0ff5e6. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

src/shared/texts.ts

@@ -0,0 +1,2 @@
+export const AI_STREAMING_ERROR_MESSAGE_PREFIX =
+  "Sorry, there was an error from the AI: ";