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/Views/ContentView.swift

@@ -29,6 +29,7 @@ struct ContentView: View {
                 LoginView()
             }
         }
+        .environment(\.locale, Locale(identifier: app.language))
         #if os(iOS)
         .frame(maxWidth: .infinity, maxHeight: .infinity)
         #else