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/_quiz_nav.dart

@@ -0,0 +1,51 @@
+import 'package:flutter/material.dart';
+import '../../theme.dart';
+
+class QuizNav extends StatelessWidget {
+  final int step;
+  final int total;
+  final VoidCallback onBack;
+  final String? skipLabel;
+  final VoidCallback? onSkip;
+
+  const QuizNav({
+    super.key,
+    required this.step,
+    required this.total,
+    required this.onBack,
+    this.skipLabel,
+    this.onSkip,
+  });
+
+  @override
+  Widget build(BuildContext context) {
+    return Padding(
+      padding: const EdgeInsets.fromLTRB(16, 12, 16, 0),
+      child: Row(
+        children: [
+          GestureDetector(
+            onTap: onBack,
+            child: Container(
+              width: 36, height: 36,
+              decoration: BoxDecoration(
+                color: SColors.bgSurface,
+                borderRadius: BorderRadius.circular(10),
+                border: Border.all(color: SColors.borderLight, width: 1.5),
+              ),
+              child: const Icon(Icons.arrow_back_ios_new_rounded, size: 15, color: SColors.textBody),
+            ),
+          ),
+          const Spacer(),
+          Text('$step of $total', style: SText.caption(SColors.textSub).copyWith(fontWeight: FontWeight.w600)),
+          if (onSkip != null) ...[
+            const SizedBox(width: 16),
+            GestureDetector(
+              onTap: onSkip,
+              child: Text(skipLabel ?? 'Skip', style: SText.caption(SColors.textSub)),
+            ),
+          ],
+        ],
+      ),
+    );
+  }
+}