refactor: move divina cover reader to native containers

[everpcpc]

Problem

The DIVINA cover reader was still driven by a SwiftUI cover path, which left it exposed to stale loading state, unnecessary slot refreshes, and first-transition animation glitches that did not affect the native scroll and page-curl readers.

Approach

Replace the cover-mode presentation with native UIKit and AppKit container implementations that own deck state, gestures, and animations directly. Reuse shared native paged page-content views so cover, scroll, and page-curl paths render through the same lower-level page presentation code, then tighten slot updates so stable-state refreshes do not reconfigure unchanged slots.

Scope

• replace the old SwiftUI DIVINA cover implementation with native cover containers on iOS and macOS
• share native paged page-content views between cover and paged readers
• fix stale loading indicator behavior and first-transition animation issues on iOS and macOS
• bump the build version to 396

Validation

• make format
• make build