Add react-native-mantine - #1257

Adds react-native-mantine to the UI section.

• What it is: a Mantine-style component library for React Native with 118 themeable, accessible components (layout, inputs, overlays, navigation, feedback, data display), a Mantine-compatible theme system, and a useForm hook.
• Platforms: iOS, Android, and web (React Native Web). Expo and bare React Native.
• Maintained: actively developed, latest release v0.20.0, works with React Native 0.81 and Expo 54.
• Docs and live demo: https://auronsan.github.io/react-native-mantine/

Disclosure: I am the author.

🤖 Generated with Claude Code