fix: trigger initial divina scroll preload on iOS (#723)

## Problem
On iOS, opening the DIVINA reader in scroll mode on the first page could
leave the second page outside the initial preload window. The first
manual page turn then had to fetch and decode that page on demand, which
showed a brief loading state.

## Approach
Trigger the regular page preload pass immediately after the initial
scroll position is synchronized in the iOS scroll reader. This keeps the
fix on the existing preload architecture and aligns the iOS
initialization path with the macOS path that already warms adjacent
pages at first presentation.

## Scope
- Update the iOS `ScrollPageView` initial-position sync path to force
the first preload pass once the committed item is known
- Leave visible-page prioritization and later page-change maintenance
behavior unchanged

Author: everpcpc

KMReader/Features/Reader/Views/ScrollPageView_iOS.swift

@@ -300,6 +300,10 @@
         if parent.viewModel.navigationTarget == nil {
           scheduleViewModelCommit(for: committedItem)
         }
+        Task { @MainActor [weak self] in
+          guard let self, self.engine.committedItem == committedItem else { return }
+          await self.parent.viewModel.preloadPages(bypassThrottle: true)
+        }
         return true
       }