🎨 Palette: Frontmatter Editor Accessibility enhancements

[threehymns]

Palette: UX improvements to the Frontmatter Editor

💡 What:

• Added aria-labels to dynamically generated property keys and inputs.
• Wrapped the icon-only "remove property" button in a Tooltip and supplied an aria-label.
• Added the asChild prop to TooltipTrigger and PopoverTrigger components that were wrapping Button components to resolve nested button HTML validation issues.

🎯 Why:
To make the frontmatter editor more accessible for screen reader users and prevent hydration errors / invalid HTML nesting within Radix UI components.

♿ Accessibility:
Improved keyboard accessibility and screen reader support through descriptive ARIA labels. Fixed button-within-button anti-pattern.

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PR created automatically by Jules for task 640626950116412853 started by @threehymns

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