feat: video letterboxing, card badge styling, component updates

- Add object-fit: contain + black background for video in DTMediaFrame
- Style .dt-card-badge with mode-colored background, padding, and
  bevel-aware positioning (negative right offset for clean diagonal clip)
- Restore DTStaggerContainer in react-native (was incorrectly deleted)
- Update DTGallery, DTModal, DTMobileFilterOverlay, DTMediaFrame components
- Refresh showcase pages for desktop and mobile

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/react-native/src/components/DTButton.tsx

@@ -243,6 +243,5 @@ const styles = StyleSheet.create({
   label: {
     fontWeight: '600',
     letterSpacing: 1,
-    textTransform: 'uppercase',
   },
 });