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/screens/onboarding/s13_referral.dart

@@ -0,0 +1,101 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_animate/flutter_animate.dart';
+import '../../theme.dart';
+import '../../widgets/s_card.dart';
+import '../../widgets/s_button.dart';
+import '../../widgets/s_progress_bar.dart';
+import '../../model/quiz_state.dart';
+import '../../router.dart';
+import 's14_analysing.dart';
+import '_quiz_nav.dart';
+
+class ReferralScreen extends StatefulWidget {
+  final QuizState state;
+  const ReferralScreen({super.key, required this.state});
+  @override
+  State<ReferralScreen> createState() => _State();
+}
+
+class _State extends State<ReferralScreen> {
+  String _selected = '';
+
+  static const _opts = [
+    (Icons.people_rounded, 'Friend or family', 'They recommended it'),
+    (Icons.play_circle_rounded, 'TikTok / Instagram', 'Saw it on social media'),
+    (Icons.search_rounded, 'Google search', 'Found it myself'),
+    (Icons.star_rounded, 'App Store', 'Featured or reviewed'),
+    (Icons.more_horiz_rounded, 'Other', 'Somewhere else'),
+  ];
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      backgroundColor: SColors.bgSurface,
+      body: SafeArea(
+        child: Column(
+          children: [
+            QuizNav(step: 6, total: 7, onBack: () => Navigator.of(context).pop()),
+            const SizedBox(height: 10),
+            Padding(padding: const EdgeInsets.symmetric(horizontal: 16), child: SProgressBar(value: 6 / 7)),
+            Expanded(
+              child: SingleChildScrollView(
+                padding: const EdgeInsets.fromLTRB(16, 24, 16, 0),
+                child: Column(
+                  crossAxisAlignment: CrossAxisAlignment.start,
+                  children: [
+                    Padding(
+                      padding: const EdgeInsets.symmetric(horizontal: 8),
+                      child: Column(
+                        crossAxisAlignment: CrossAxisAlignment.start,
+                        children: [
+                          Text('How did you\nhear about us?', style: SText.headline2(SColors.textHead))
+                              .animate().fadeIn(duration: 350.ms),
+                          const SizedBox(height: 6),
+                          Text("Helps us understand where our community comes from.", style: SText.subtitle(SColors.textSub))
+                              .animate().fadeIn(duration: 350.ms, delay: 80.ms),
+                          const SizedBox(height: 20),
+                        ],
+                      ),
+                    ),
+                    ..._opts.asMap().entries.map((e) {
+                      final i = e.key; final (icon, label, sub) = e.value;
+                      return Padding(
+                        padding: const EdgeInsets.only(bottom: 8),
+                        child: SCard(
+                          icon: icon, label: label, sub: sub,
+                          isSelected: _selected == label,
+                          onTap: () => setState(() => _selected = label),
+                        ).animate().fadeIn(duration: 280.ms, delay: (100 + i * 55).ms)
+                         .slideX(begin: 0.03, end: 0, duration: 280.ms),
+                      );
+                    }),
+                  ],
+                ),
+              ),
+            ),
+            Padding(
+              padding: const EdgeInsets.fromLTRB(16, 12, 16, 20),
+              child: Column(
+                children: [
+                  SButton(
+                    label: 'BUILD MY REPORT',
+                    enabled: _selected.isNotEmpty,
+                    onTap: _selected.isNotEmpty ? () {
+                      widget.state.referral = _selected;
+                      Navigator.of(context).push(slideRoute(AnalysingScreen(state: widget.state)));
+                    } : null,
+                  ),
+                  const SizedBox(height: 12),
+                  GestureDetector(
+                    onTap: () => Navigator.of(context).push(slideRoute(AnalysingScreen(state: widget.state))),
+                    child: Text('Skip', style: SText.caption(SColors.textSub)),
+                  ),
+                ],
+              ),
+            ),
+          ],
+        ),
+      ),
+    );
+  }
+}