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_button.dart

@@ -0,0 +1,88 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import '../theme.dart';
+
+// Duolingo-style pressable button.
+// Stack of two containers: dark "shadow" bottom + colored top that translates down on press.
+class SButton extends StatefulWidget {
+  final String label;
+  final VoidCallback? onTap;
+  final bool enabled;
+  final Color? color;
+  final Color? shadowColor;
+  final Color? textColor;
+
+  const SButton({
+    super.key,
+    required this.label,
+    required this.onTap,
+    this.enabled = true,
+    this.color,
+    this.shadowColor,
+    this.textColor,
+  });
+
+  @override
+  State<SButton> createState() => _SButtonState();
+}
+
+class _SButtonState extends State<SButton> {
+  bool _pressed = false;
+
+  bool get _active => widget.enabled && widget.onTap != null;
+
+  static const _shadowH = 4.0;
+  static const _btnH = 52.0;
+  static const _radius = 16.0;
+
+  @override
+  Widget build(BuildContext context) {
+    final bg = _active ? (widget.color ?? SColors.primary) : const Color(0xFFE5E5E5);
+    final shadow = _active ? (widget.shadowColor ?? SColors.primaryDark) : const Color(0xFFB8B8B8);
+    final textCol = _active ? (widget.textColor ?? Colors.white) : SColors.textDisabled;
+    final offset = (_pressed && _active) ? _shadowH : 0.0;
+
+    return GestureDetector(
+      onTapDown: (_) { if (_active) setState(() => _pressed = true); },
+      onTapUp: (_) {
+        if (!_active) return;
+        setState(() => _pressed = false);
+        HapticFeedback.mediumImpact();
+        widget.onTap!();
+      },
+      onTapCancel: () { if (_active) setState(() => _pressed = false); },
+      child: SizedBox(
+        height: _btnH + _shadowH,
+        child: Stack(
+          children: [
+            // Shadow layer (always at bottom)
+            Positioned.fill(
+              child: Container(
+                decoration: BoxDecoration(
+                  color: shadow,
+                  borderRadius: BorderRadius.circular(_radius),
+                ),
+              ),
+            ),
+            // Button layer (translates down on press, "sinking" into shadow)
+            AnimatedPositioned(
+              duration: const Duration(milliseconds: 80),
+              top: offset,
+              left: 0,
+              right: 0,
+              height: _btnH,
+              child: Container(
+                decoration: BoxDecoration(
+                  color: bg,
+                  borderRadius: BorderRadius.circular(_radius),
+                ),
+                alignment: Alignment.center,
+                child: Text(widget.label, style: SText.button(textCol)),
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}