Add chapters, history, bookmarks, live download progress, and i18n

- Chapter navigation with auto-scroll to current chapter and end-of-chapter sleep timer
- Opt-in listening history (local-only) with XML export and per-item quick menu
- Bookmarks with server sync via Audiobookshelf API
- Live MB counter during downloads via URLSessionDownloadTask delegate
- In-progress downloads shown in "Heruntergeladen" with dimmed cover + ring overlay
- Cover image cache (50 MB memory / 500 MB disk URLCache)
- German/English localization (de.lproj, en.lproj)
- Loading spinner now triggers immediately on view switch instead of waiting for the network

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ABS Client/Audiobookshelf swift/Models/Models.swift

@@ -1,5 +1,12 @@
 import Foundation
 
+struct Chapter: Codable, Identifiable, Hashable {
+    let id: Int
+    let start: Double
+    let end: Double
+    let title: String
+}
+
 struct Library: Codable, Identifiable, Hashable {
     let id: String
     let name: String
@@ -15,6 +22,7 @@ struct LibraryItem: Codable, Identifiable, Hashable {
     var mediaType: String = "book"
     var episodeId: String? = nil
     var description: String? = nil
+    var chapters: [Chapter] = []
 
     var isPodcast: Bool { mediaType == "podcast" }
     var isPodcastContainer: Bool { isPodcast && episodeId == nil }