Read screen size with useWindowDimensions, not at module scope

Phones rotate, tablets split the screen and foldables change size while your app is open. A width read once at import time with Dimensions.get('window') goes stale and your layout breaks.

• Use useWindowDimensions() inside components so they re-render when the size changes.
• Prefer flexbox and percentage layouts over computed pixel widths where you can.
• Handle safe areas (notches, home indicators, system bars) with react-native-safe-area-context so content is not drawn under them. This matters more now that Android apps draw edge to edge.