🎨 Palette: [UX improvement] Add global focus-visible state for keyboard accessibility

[mapleleaflatte03]

🎨 Palette: [UX improvement] Add global focus-visible state for keyboard accessibility

💡 What: Added a global :focus-visible CSS rule to provide a clear focus ring using the primary accent color.
🎯 Why: Keyboard navigation lacked visual feedback, making the UI inaccessible for non-mouse users.
📸 Before/After: Verified via Playwright screenshot, focus ring clearly visible on interactive elements.
♿ Accessibility: Significantly improves WCAG compliance for focus indicators (Success Criterion 2.4.7 Focus Visible).

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PR created automatically by Jules for task 9731888300487766407 started by @mapleleaflatte03

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