🎨 Palette: Enhance Scratchpad Accessibility and UX

[google-labs-jules]

💡 What: Added aria-label, improved placeholder text, and added :focus styles to the Scratchpad webview.
🎯 Why: To make the Scratchpad accessible to screen readers, provide better guidance on data persistence, and ensure keyboard navigation visibility.
📸 Before/After: Added focus border and descriptive placeholder.
♿ Accessibility: Added aria-label and visible focus indicator.

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PR created automatically by Jules for task 8019501590667766634 started by @Sisyphean-a

[google-labs-jules]

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