fix(reader): correct divina scroll page jump on app background/foreground

[everpcpc]

Problem

Minimizing and reopening the app while reading in Divina scroll mode causes the reader to jump backward by 2 pages (or 1 page from even-numbered pages, then 2 on subsequent cycles).

Approach

The root cause is in centeredItem() which calculates the viewport center to find the nearest page. For UIScrollView, bounds.origin equals contentOffset, so bounds.midX already includes the content offset. The old code added contentOffset again, producing a center at 2 * contentOffset + width/2 instead of the correct contentOffset + width/2. This biased the nearest-item lookup toward wrong pages whenever multiple items were momentarily visible during app state transitions.

Additionally, the willResignActive notification handler dispatched via Task { @MainActor } which yields before executing, allowing the collection view state to change before the position was committed.

Scope

• Fix centeredItem() center calculation to use bounds.midX/midY directly (matching the macOS implementation)
• Switch notification handler from async Task to synchronous MainActor.assumeIsolated

Testing

• iOS build passes
• macOS build passes
• Manual: read in Divina scroll mode, minimize and reopen — position should be preserved