🎨 Palette: Improve Button Loading State

[google-labs-jules]

💡 What

Replaced the full-screen loading overlay with a more focused loading state directly on the "Generate Poster" button. The button now disables and its text changes to "Generating..." during the poster creation process.

🎯 Why

The previous full-screen overlay was a disruptive user experience, creating a "double loading" effect. This change provides clear, immediate feedback at the point of interaction, making the UI feel more responsive and less jarring.

📸 Before/After

Before: A full-screen loading overlay would appear, obscuring the entire interface.
After: The "Generate Poster" button now shows an inline loading state, providing a more modern and less intrusive user experience. The screenshot from the verification process shows the new state.

♿ Accessibility

This change improves the user experience by providing clear and immediate feedback without a disorienting full-screen overlay. The button's disabled state also prevents multiple, accidental submissions.

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PR created automatically by Jules for task 7686536949935516382 started by @anaxifty

[google-labs-jules]

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