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/main.dart

@@ -1,7 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'theme.dart';
-import 'screens/splash_screen.dart';
+import 'screens/onboarding/s01_splash.dart';
 
 void main() {
   WidgetsFlutterBinding.ensureInitialized();
@@ -21,7 +21,7 @@ class ShayneeApp extends StatelessWidget {
       title: 'Shaynee',
       debugShowCheckedModeBanner: false,
       theme: buildTheme(),
-      home: const SplashScreen(),
+      home: const SplashScreen(), // s01
     );
   }
 }