fix: repair animated playback in scroll reader

[everpcpc]

Problem

Animated image pages in the scroll reader could behave inconsistently. Tap-based navigation would start inline playback, but drag-based page changes only updated the committed item and left visible page views on a stale playback state. The temporary animated poster path also bypassed the normal fitted image presentation, so placeholders could render with the wrong geometry before playback began.

Approach

Route playback state changes through the same native page reconfiguration path used for normal page rendering, instead of mutating only the animated source URL on existing views. Keep the static fitted image view visible as the poster placeholder while the animated layer starts, so the placeholder continues to inherit the established fit layout.

Scope

• Reconfigure native paged cells on playback activation changes for iOS and macOS scroll reader implementations
• Remove the separate poster layer-copy path in native page items and simplify the animated presentation helper
• Delete unused focused animated playback state and related no-op calls from the reader view model flow

Validation

• make format
• make build-macos
• make build-ios
• make build-tvos