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/model/quiz_state.dart

@@ -0,0 +1,8 @@
+class QuizState {
+  String name = '';
+  String skinType = '';
+  String age = '';
+  List<String> concerns = [];
+  String goal = '';
+  String referral = '';
+}