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/Audiobookshelf_swiftApp.swift

@@ -8,11 +8,24 @@ struct Audiobookshelf_swiftApp: App {
     @State private var appState = AppState()
 
     init() {
+        configureImageCache()
         #if os(iOS)
         configureAudioSession()
         #endif
     }
 
+    /// Increases the shared URLCache so AsyncImage covers persist across renders
+    /// and app launches. Default capacity is tiny (~4 MB / 20 MB).
+    private func configureImageCache() {
+        let memoryCapacity = 50 * 1024 * 1024      // 50 MB
+        let diskCapacity   = 500 * 1024 * 1024     // 500 MB
+        URLCache.shared = URLCache(
+            memoryCapacity: memoryCapacity,
+            diskCapacity: diskCapacity,
+            diskPath: "covers"
+        )
+    }
+
     var body: some Scene {
         #if os(macOS)
         WindowGroup {