Fix VoiceOver performance lag with large libraries

[matalvernaz]

Problem

When VoiceOver is enabled and a large number of books are loaded, the library list becomes noticeably laggy. Three separate issues compound to cause this:

1. DynamicAccessibilityLabelModifier recomputes labels for every item on track change — the onChange(of: playerManager.currentItem?.relativePath) handler calls setupObserver() unconditionally on every item in the list. For non-playing items this synchronously calls VoiceOverService.getAccessibilityLabel(), which does time arithmetic and localizedStringWithFormat with multiple arguments. With 100+ books this fires O(n) times on the main thread every time playback starts/stops/changes.

2. Custom accessibility rotors call getAccessibilityLabel for all items on every rebuild — customBookRotor and customFolderRotor pass VoiceOverService.getAccessibilityLabel(for: item) as the rotor entry label for every item. Rotors are rebuilt whenever filteredResults changes, and VoiceOver triggers these rebuilds frequently during navigation.

3. immediateProgressUpdatePublisher in ItemProgressView is unthrottled — the folderProgressUpdated subscription is throttled to 1s but immediateProgressUpdatePublisher is not, so rapid-fire progress writes hit every visible cell immediately.

Changes

• DynamicAccessibilityLabelModifier: skip setupObserver() in the onChange handler for items that are neither currently playing nor have an active subscription. The expensive label recomputation now only runs for the one item that just started or stopped playing.

• ItemListView custom rotors: use item.title as the rotor entry label instead of calling getAccessibilityLabel(). The full rich label (with remaining time, progress %) is already applied by dynamicAccessibilityLabel() on each row — the rotor list only needs the title for navigation purposes.

• ItemProgressView: add .throttle(for: .seconds(1), scheduler: DispatchQueue.main, latest: true) to the immediateProgressUpdatePublisher subscription, consistent with the existing throttle on folderProgressUpdated.

Testing

• Load 50+ books in the library
• Enable VoiceOver
• Navigate the list with swipe gestures — scrolling and focus changes should be noticeably smoother
• Start/pause/change playback — verify the playing item's label still updates with remaining time and the non-playing items are unaffected
• Use the Books and Folders rotors — verify items are correctly navigable by title