Rename app to Nook (display name only)

Changes the user-visible name from "ABS Client" to "Nook" across the
iOS home screen, macOS Dock/menu bar, login header, sidebar header, and
README. Bundle ID (com.local.ABS-Client), keychain key, and background
download session ID are intentionally left unchanged so the existing
App Store listing (ID 6770326901) updates in place — installed users
keep the app, stored credentials, and downloads.

- Build settings: INFOPLIST_KEY_CFBundleDisplayName, CFBundleName,
  PRODUCT_NAME → "Nook" (Debug + Release); built app is now Nook.app
- Info-iOS.plist: CFBundleDisplayName + CFBundleName → "Nook"
- Localizable.strings (de + en): login.title + sidebar.app_title → "Nook"
- LoginView: hardcoded headers → "Nook"
- README: branding text → "Nook" (git repo URLs and App Store URL
  unchanged — repo and listing rename happen separately)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ABS Client/Audiobookshelf swift/en.lproj/Localizable.strings

@@ -125,7 +125,7 @@
 "library.settings" = "Settings";
 
 /* Login */
-"login.title" = "ABS Client";
+"login.title" = "Nook";
 "login.server_placeholder" = "https://my-server.com";
 "login.username" = "Username";
 "login.password" = "Password";
@@ -141,7 +141,7 @@
 "sidebar.offline" = "Offline";
 "sidebar.history" = "History";
 "sidebar.logout" = "Sign out";
-"sidebar.app_title" = "ABS Client";
+"sidebar.app_title" = "Nook";
 "sidebar.status_online" = "Online";
 "sidebar.status_offline" = "Offline";