fix(example): pin demo colors so the screen is readable in dark mode

The demo hardcoded light-theme colors with no explicit background, so on a
device in dark mode the title and input text rendered dark-on-dark.

example/src/App.tsx

@@ -138,14 +138,23 @@ function Field({
         onChangeText={onChangeText}
         autoCapitalize="none"
         autoCorrect={false}
+        placeholderTextColor="#999"
       />
     </View>
   );
 }
 
 const styles = StyleSheet.create({
-  container: { padding: 24, gap: 16, paddingTop: 72 },
-  title: { fontSize: 28, fontWeight: '700', marginBottom: 8 },
+  // Colors are pinned rather than theme-aware: the SDK's own UI is light, so a
+  // dark demo shell around it would be inconsistent anyway.
+  container: {
+    padding: 24,
+    gap: 16,
+    paddingTop: 72,
+    backgroundColor: '#fff',
+    flexGrow: 1,
+  },
+  title: { fontSize: 28, fontWeight: '700', marginBottom: 8, color: '#111' },
   field: { gap: 6 },
   label: { fontSize: 13, color: '#555' },
   input: {
@@ -155,6 +164,8 @@ const styles = StyleSheet.create({
     paddingHorizontal: 12,
     paddingVertical: 10,
     fontSize: 15,
+    color: '#111',
+    backgroundColor: '#fff',
   },
   row: { flexDirection: 'row', gap: 8 },
   chip: {
@@ -178,6 +189,6 @@ const styles = StyleSheet.create({
   buttonText: { color: '#fff', fontSize: 16, fontWeight: '600' },
   error: { color: '#c0392b', fontSize: 14 },
   result: { gap: 12, alignItems: 'center' },
-  resultCode: { fontSize: 16, fontWeight: '600' },
+  resultCode: { fontSize: 16, fontWeight: '600', color: '#111' },
   portrait: { width: 180, height: 180, borderRadius: 12 },
 });