example: overhaul App.tsx to use hooks-based playground UI

Replace the legacy sample with a simplified "Sensitive Info Playground"
example that leverages useSecureStorage and useSecurityAvailability.
Clean up imports, reduce access-control options to 'open' and 'biometric',
wire up save/reveal/delete/clear/refresh handlers, update ActionButton
behaviour, and refresh styles/layout to match the streamlined UI.

Author: mCodex