fix: fallback to useEffect when no navigation library is available

[malopezr7]

Problem

Projects that don't use expo-router or @react-navigation/native crash on import with:

Error: No useFocusEffect implementation found

This makes react-native-grab unusable in any project that uses a custom navigation solution.

Solution

Instead of throwing, fall back to React's useEffect as a best-effort substitute. The callback runs on mount instead of on screen focus, which is acceptable for:

• Single-screen setups
• Custom navigators without a focus/blur lifecycle
• Projects that only use ReactNativeGrabRoot without ReactNativeGrabScreen

Changes

• src/react-native/grab-screen.tsx: Replace throw new Error(...) with a useEffect-based fallback in getFocusEffectImpl()

Test plan

• Verified in a React Native 0.83 project using a custom router (no expo-router or react-navigation)
• ReactNativeGrabRoot renders correctly without crash
• No regression when @react-navigation/native is installed (existing try/catch still resolves first)