style: fix login screen form overlap with logo on smaller devices

app/login.tsx

@@ -149,7 +149,7 @@ export default function LoginScreen() {
           >
             {/* 底部表单区域 */}
             <View
-              style={[styles.bottom, { paddingBottom: insets.bottom + 70 }]}
+              style={[styles.bottom, { paddingBottom: insets.bottom + 20 }]}
             >
               {/* 表单 */}
               <View style={styles.form}>