feat: landing page entrance animations

[stephenhungg]

Summary

• adds spring-scale entrance animation on the replate logo with a subtle breathing pulse after it settles
• staggered fade-in-up for welcome text and subtitle
• staggered fade-in-down for login/signup buttons
• all animations use react-native-reanimated (already installed) and run on the native thread

Test plan

• open the app logged out and verify the landing page animation sequence plays smoothly
• verify the logo spring bounce feels natural and the pulse is subtle
• verify text and buttons stagger in with correct timing
• test on both iOS and Android