🎨 Palette: Improve Dropzone Accessibility and Keyboard Interactions

[Nawayisus]

💡 What: Added keyboard accessibility to the #dropZone element, making it focusable and mapping Enter/Space and general click interactions to open the file selection dialog. Mapped visual highlight functions to focus and blur events.
🎯 Why: File upload dropzones are often inaccessible to keyboard users and typically only the inner button is clickable. This change makes the entire dropzone interactive and keyboard navigable, significantly improving usability.
♿ Accessibility: Added tabindex="0", role="button", aria-label, and full keyboard interaction handlers (keydown for Enter/Space).
📸 Before/After: Verified visually via Playwright tests proving the new interactions apply the correct visual bg-white classes during focus.

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PR created automatically by Jules for task 18313325538136140779 started by @Nawayisus

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