feat: complete 20-screen onboarding flow

Full quiz flow s01-s20: splash, intro, value prop, social proof,
name/skin-type/age/concerns/goal quiz with insights, analysing loader,
result with skin score, AM/PM routines, ingredient science, signup, welcome.

Duolingo-style design: flat cards, pressable SButton (Stack trick),
4px progress bar, mascot moods (wave/think/analyze/celebrate),
confetti on result+welcome, personalized content throughout.

lib/widgets/confetti.dart

@@ -0,0 +1,100 @@
+import 'dart:math';
+import 'package:flutter/material.dart';
+
+class ConfettiOverlay extends StatefulWidget {
+  const ConfettiOverlay({super.key});
+
+  @override
+  State<ConfettiOverlay> createState() => _ConfettiOverlayState();
+}
+
+class _ConfettiOverlayState extends State<ConfettiOverlay>
+    with SingleTickerProviderStateMixin {
+  late AnimationController _ctrl;
+  final _rand = Random();
+  late List<_Particle> _particles;
+
+  @override
+  void initState() {
+    super.initState();
+    _particles = List.generate(40, (_) => _Particle(_rand));
+    _ctrl = AnimationController(
+      vsync: this,
+      duration: const Duration(milliseconds: 2400),
+    )..forward();
+  }
+
+  @override
+  void dispose() {
+    _ctrl.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return AnimatedBuilder(
+      animation: _ctrl,
+      builder: (_, __) => CustomPaint(
+        painter: _ConfettiPainter(_particles, _ctrl.value),
+        size: Size.infinite,
+      ),
+    );
+  }
+}
+
+class _Particle {
+  final double x;
+  double y;
+  final double speed;
+  final double size;
+  final Color color;
+  final double rotation;
+  final double rotSpeed;
+
+  _Particle(Random r)
+      : x = r.nextDouble(),
+        y = -0.1 - r.nextDouble() * 0.3,
+        speed = 0.3 + r.nextDouble() * 0.5,
+        size = 6 + r.nextDouble() * 8,
+        color = [
+          const Color(0xFF5B4FCF),
+          const Color(0xFFFF7EB3),
+          const Color(0xFFFFD700),
+          const Color(0xFF4CAF50),
+          const Color(0xFF1CB0F6),
+          const Color(0xFFFF7043),
+        ][r.nextInt(6)],
+        rotation = r.nextDouble() * 2 * pi,
+        rotSpeed = (r.nextDouble() - 0.5) * 6;
+}
+
+class _ConfettiPainter extends CustomPainter {
+  final List<_Particle> particles;
+  final double t;
+
+  _ConfettiPainter(this.particles, this.t);
+
+  @override
+  void paint(Canvas canvas, Size size) {
+    for (final p in particles) {
+      final y = (p.y + t * p.speed) * size.height;
+      if (y > size.height) continue;
+      final x = p.x * size.width;
+      final rot = p.rotation + t * p.rotSpeed;
+
+      canvas.save();
+      canvas.translate(x, y);
+      canvas.rotate(rot);
+
+      final paint = Paint()..color = p.color;
+      canvas.drawRect(
+        Rect.fromCenter(center: Offset.zero, width: p.size, height: p.size * 0.5),
+        paint,
+      );
+      canvas.restore();
+    }
+  }
+
+  @override
+  bool shouldRepaint(_ConfettiPainter old) => old.t != t;
+}