🎨 Palette: Add error handling for poster image loading

[google-labs-jules]

💡 What

Added an onerror handler to the poster image loading logic. If the generated poster fails to load, the loading overlay is now hidden and a clear error toast is displayed.

🎯 Why

Previously, if the image failed to load after a successful API call, the user would be trapped on an infinite loading screen with no feedback. This change prevents the UI from becoming unresponsive and informs the user about the failure.

📸 Before/After

Before: A perpetual loading spinner if the image failed to load.
After: The loading spinner is hidden and an error message is displayed, as seen in the verification screenshot.

♿ Accessibility

This change improves the experience for all users by providing crucial feedback on system status, a core principle of accessibility. It ensures users are not left in a state of confusion or forced to abandon the application.

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PR created automatically by Jules for task 7203415602493381604 started by @anaxifty

[google-labs-jules]

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