redesign: duolingo-pattern onboarding, no gradient, pressable button

- Nunito font replacing Plus Jakarta Sans
- SButton: Stack-based 3D pressable (dark shadow layer + top layer translates on press)
- SCard: flat border-based selection, no shadows
- SProgressBar: 4px thin bar, animated fill
- All screens: white bg, no gradients
- Duolingo layout: illustration top zone, content below, sticky button

lib/widgets/s_progress_bar.dart

@@ -0,0 +1,38 @@
+import 'package:flutter/material.dart';
+import '../theme.dart';
+
+class SProgressBar extends StatelessWidget {
+  final double value; // 0.0 to 1.0
+
+  const SProgressBar({super.key, required this.value});
+
+  @override
+  Widget build(BuildContext context) {
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        return Stack(
+          children: [
+            Container(
+              height: 4,
+              width: constraints.maxWidth,
+              decoration: BoxDecoration(
+                color: SColors.borderLight,
+                borderRadius: BorderRadius.circular(2),
+              ),
+            ),
+            AnimatedContainer(
+              duration: const Duration(milliseconds: 300),
+              curve: Curves.easeOut,
+              height: 4,
+              width: constraints.maxWidth * value.clamp(0.0, 1.0),
+              decoration: BoxDecoration(
+                color: SColors.primary,
+                borderRadius: BorderRadius.circular(2),
+              ),
+            ),
+          ],
+        );
+      },
+    );
+  }
+}