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/router.dart

@@ -0,0 +1,27 @@
+import 'package:flutter/material.dart';
+
+Route<T> slideRoute<T>(Widget page) => PageRouteBuilder<T>(
+  pageBuilder: (_, a, __) => page,
+  transitionsBuilder: (_, a, __, child) => SlideTransition(
+    position: Tween<Offset>(begin: const Offset(1, 0), end: Offset.zero)
+        .animate(CurvedAnimation(parent: a, curve: Curves.easeOut)),
+    child: child,
+  ),
+  transitionDuration: const Duration(milliseconds: 320),
+);
+
+Route<T> fadeRoute<T>(Widget page) => PageRouteBuilder<T>(
+  pageBuilder: (_, a, __) => page,
+  transitionsBuilder: (_, a, __, child) => FadeTransition(opacity: a, child: child),
+  transitionDuration: const Duration(milliseconds: 400),
+);
+
+Route<T> scaleRoute<T>(Widget page) => PageRouteBuilder<T>(
+  pageBuilder: (_, a, __) => page,
+  transitionsBuilder: (_, a, __, child) => ScaleTransition(
+    scale: Tween<double>(begin: 0.88, end: 1.0)
+        .animate(CurvedAnimation(parent: a, curve: Curves.easeOut)),
+    child: FadeTransition(opacity: a, child: child),
+  ),
+  transitionDuration: const Duration(milliseconds: 350),
+);