Create a mobile app design system for [APP TYPE] on [iOS / ANDROID / BOTH].
Provide:
1. **Design Principles**: 5 guiding principles for the app's design
2. **Color System**: Primary, secondary, semantic, surface colors (light + dark mode)
3. **Typography Scale**: Type ramp following platform conventions (SF Pro / Roboto / custom)
4. **Spacing & Grid**: 4pt/8pt grid system, safe areas, margins
5. **Component Library**: Specs for buttons, inputs, cards, lists, navigation bars, tab bars, modals, sheets, toast notifications
6. **Navigation Patterns**: Tab bar, drawer, stack — when to use each
7. **Gesture Conventions**: Swipe, long-press, pull-to-refresh — consistent behaviors
8. **Animation Tokens**: Durations, easing curves, transition types
9. **Platform Differences**: Where iOS and Android conventions diverge
10. **Accessibility**: Touch targets (44pt min), Dynamic Type support, VoiceOver/TalkBack