Fix a11y issues

[T-kesh]

• Updated AddressDisplay copy button to be focus-visible and always partially visible (opacity-50) for keyboard & mobile users, with proper ARIA label.
• Implemented SorobanPanel with an accessible textarea, linking label via htmlFor/id and adding state handling.
• Refactored ErrorBoundary to expose raw error messages only in development; production now shows a generic “See the browser console for details.” message, preserving UX and security.

All changes pass npm run build and npm run lint. Accessibility criteria WCAG 1.3.1, 1.4.3, and 2.4.7 are addressed.
closes #27

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