iOS: replace shuffle/repeat with time skip and add chapter navigation to full player

- Leftmost button: 15-second backward skip (gobackward.15)
- Second button: previous chapter navigation (backward.end.fill)
- Center: play/pause (unchanged)
- Fourth button: next chapter navigation (forward.end.fill)
- Rightmost button: 15-second forward skip (goforward.15)
- Added prevChapter tracking in AudioPlayerService
- Added skipToPrevChapter notification support
- Chapter nav buttons disabled when no prev/next available (0.5 opacity)
- Updated load() signature to accept prevChapter parameter
- Auto-next properly tracks prevChapter when advancing chapters

ios/LibNovel/LibNovel/Views/ChapterReader/ChapterReaderView.swift

@@ -61,6 +61,14 @@ struct ChapterReaderView: View {
                 vm.navigateTo = next
             }
         }
+        .onReceive(NotificationCenter.default.publisher(for: .skipToNextChapter)) { note in
+            guard let next = note.userInfo?["next"] as? Int else { return }
+            vm.navigateTo = next
+        }
+        .onReceive(NotificationCenter.default.publisher(for: .skipToPrevChapter)) { note in
+            guard let prev = note.userInfo?["prev"] as? Int else { return }
+            vm.navigateTo = prev
+        }
         .navigationDestination(isPresented: Binding(
             get: { vm.navigateTo != nil },
             set: { if !$0 { vm.navigateTo = nil } }