Android: Janky full screen reveal animations

[churnish]

Smooth as butter on iOS, janky navbar/header slide in animation on Android.

[churnish]

Fixed in 3d1515e. Android WebView's single-threaded compositor makes CSS transitions during active scroll inherently janky — replaced with WAAPI element.animate() for header and navbar. Also: reverse bridge on show path (eliminates 25ms forced layout from scrollTop), split CSS rules for iOS/Android, will-change pre-promotion.

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