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_card.dart

@@ -0,0 +1,111 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import '../theme.dart';
+
+// Duolingo-style option card. Flat — no shadow, border-based selection.
+class SCard extends StatelessWidget {
+  final IconData icon;
+  final String label;
+  final String sub;
+  final bool isSelected;
+  final VoidCallback onTap;
+
+  const SCard({
+    super.key,
+    required this.icon,
+    required this.label,
+    required this.isSelected,
+    required this.onTap,
+    this.sub = '',
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTap: () {
+        HapticFeedback.selectionClick();
+        onTap();
+      },
+      child: AnimatedContainer(
+        duration: const Duration(milliseconds: 150),
+        curve: Curves.easeOut,
+        height: sub.isEmpty ? 62 : null,
+        padding: sub.isEmpty
+            ? const EdgeInsets.symmetric(horizontal: 14)
+            : const EdgeInsets.symmetric(horizontal: 14, vertical: 14),
+        decoration: BoxDecoration(
+          color: isSelected ? SColors.primaryLight : SColors.bgSurface,
+          borderRadius: BorderRadius.circular(12),
+          border: Border.all(
+            color: isSelected ? SColors.primaryBorder : SColors.borderLight,
+            width: isSelected ? 2.0 : 1.5,
+          ),
+        ),
+        child: Row(
+          children: [
+            // Icon box
+            AnimatedContainer(
+              duration: const Duration(milliseconds: 150),
+              width: 40,
+              height: 40,
+              decoration: BoxDecoration(
+                color: isSelected
+                    ? SColors.primary.withAlpha(20)
+                    : const Color(0xFFF7F7F7),
+                borderRadius: BorderRadius.circular(10),
+              ),
+              child: Icon(
+                icon,
+                size: 22,
+                color: isSelected ? SColors.primary : SColors.textBody,
+              ),
+            ),
+            const SizedBox(width: 12),
+            // Labels
+            Expanded(
+              child: Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                crossAxisAlignment: CrossAxisAlignment.start,
+                children: [
+                  Text(
+                    label,
+                    style: SText.cardLabel(
+                      isSelected ? SColors.primary : SColors.textHead,
+                    ),
+                  ),
+                  if (sub.isNotEmpty) ...[
+                    const SizedBox(height: 2),
+                    Text(
+                      sub,
+                      style: SText.cardSub(SColors.textSub),
+                      maxLines: 1,
+                      overflow: TextOverflow.ellipsis,
+                    ),
+                  ],
+                ],
+              ),
+            ),
+            const SizedBox(width: 10),
+            // Radio
+            AnimatedContainer(
+              duration: const Duration(milliseconds: 150),
+              width: 22,
+              height: 22,
+              decoration: BoxDecoration(
+                shape: BoxShape.circle,
+                color: isSelected ? SColors.primary : Colors.transparent,
+                border: Border.all(
+                  color: isSelected ? SColors.primary : SColors.borderMedium,
+                  width: 1.5,
+                ),
+              ),
+              child: isSelected
+                  ? const Icon(Icons.check, size: 13, color: Colors.white)
+                  : null,
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}