spark 2.1.0 splat viewer tab with OS-fused gyro look-around

WebView-hosted Spark renderer (butterfly.spz demo scene), fixed camera
position, rotation driven by expo-sensors DeviceMotion (ROTATION_VECTOR /
CoreMotion xMagneticNorthZVertical) via injectJavaScript; web fallback
uses deviceorientation in an iframe. Tap to re-center yaw.

app/(tabs)/splat.tsx

@@ -0,0 +1,5 @@
+import { SplatViewer } from '@/components/splat-viewer';
+
+export default function SplatScreen() {
+  return <SplatViewer />;
+}