fix: stabilize reader presentation transitions

[everpcpc]

Problem

Divina and PDF reader overlay controls used a scaled bouncy transition that felt too springy. More importantly, rebuilding the Divina reader presentation when switching between cover, page curl, scroll, or different reading directions could lose the active reading position, causing jumps to the first or last item or leaving scroll mode waiting in an incorrect initial state.

Approach

Tighten the overlay transitions to simple move-plus-opacity animations, and move the reader presentation state toward explicit ownership in ReaderViewModel so presentation rebuilds preserve the current page. On the scroll reader path, only mark initial positioning as complete after the target item is actually visible, which lets layout retries finish correctly when transitioning in from cover or page curl.

Scope

• smooth Divina and PDF controls overlay animations
• preserve the current page across Divina presentation rebuilds
• harden iOS and macOS scroll initial-position recovery
• bump the build version to 386

Validation

• make format
• make build