🎨 Palette: Replace Full-Screen Loader with In-Button Spinner

[google-labs-jules]

💡 What: Replaced the full-screen loading overlay with an in-button spinner on the "Generate Poster" button.

🎯 Why: The previous full-screen loader was disruptive and blocked the user from viewing the rest of the UI. An in-button spinner provides more contextual and less intrusive feedback, improving the user experience. Disabling the button during the operation also prevents accidental double-clicks.

📸 Before/After:
Before: A full-screen overlay with a spinner would appear.
After: The "Generate Poster" button now shows a spinner inside of it and is disabled while the poster is being generated. A screenshot of the new state is attached.

♿ Accessibility: This change improves the user experience for all users by providing clear, immediate feedback. Disabling the button during the loading state prevents confusing interactions and ensures a more predictable interface.

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PR created automatically by Jules for task 12540048608301591097 started by @anaxifty

[google-labs-jules]

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