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/s01_splash.dart

@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_animate/flutter_animate.dart';
+import '../../theme.dart';
+import '../../widgets/mascot.dart';
+import 's02_hi_im_shaynee.dart';
+import '../../router.dart';
+
+class SplashScreen extends StatefulWidget {
+  const SplashScreen({super.key});
+  @override
+  State<SplashScreen> createState() => _State();
+}
+
+class _State extends State<SplashScreen> {
+  @override
+  void initState() {
+    super.initState();
+    Future.delayed(const Duration(milliseconds: 2600), () {
+      if (mounted) Navigator.of(context).pushReplacement(fadeRoute(const HiImShayneeScreen()));
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      backgroundColor: SColors.bgSurface,
+      body: SafeArea(
+        child: Column(
+          children: [
+            const Spacer(flex: 2),
+            Mascot(mood: MascotMood.neutral, size: 200)
+                .animate()
+                .fadeIn(duration: 700.ms, delay: 300.ms)
+                .slideY(begin: 0.1, end: 0, duration: 700.ms, curve: Curves.easeOut),
+            const SizedBox(height: 28),
+            Text('shaynee', style: SText.headline1(SColors.textHead).copyWith(fontSize: 44, letterSpacing: -1))
+                .animate().fadeIn(duration: 600.ms, delay: 600.ms),
+            const SizedBox(height: 6),
+            Text('your ai skincare companion', style: SText.subtitle(SColors.textSub))
+                .animate().fadeIn(duration: 600.ms, delay: 800.ms),
+            const Spacer(flex: 3),
+          ],
+        ),
+      ),
+    );
+  }
+}