🎨 Palette: Improve loading state on Generate button

[google-labs-jules]

💡 What: Replaced the full-screen loading overlay with a contextual loading indicator on the "Generate Poster" button.
🎯 Why: The previous full-screen overlay was disruptive to the user experience. This change provides immediate, non-blocking feedback directly at the point of interaction, making the app feel more responsive and polished.
📸 Before/After: I was unable to get a reliable "after" screenshot due to race conditions in the testing environment. Before: a full-screen semi-transparent overlay with a spinner would cover the entire page. After: The "Generate Poster" button becomes disabled, its opacity is lowered, and it displays a small spinner next to the text "Generating...".
♿ Accessibility: This change improves the user experience for all users by providing clear, contextual feedback. The button is disabled during the loading state, preventing multiple submissions and making the UI state clearer.

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PR created automatically by Jules for task 6513385169592550205 started by @anaxifty

[google-labs-jules]

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