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/Views/LoginView.swift

@@ -35,7 +35,7 @@ struct LoginView: View {
                     Image(systemName: "books.vertical.fill")
                         .font(.system(size: 52, weight: .regular))
                         .foregroundStyle(.white)
-                    Text("ABS Client")
+                    Text("Nook")
                         .font(.title.bold())
                         .foregroundStyle(.white)
                     Text("Verbinde dich mit deinem Audiobookshelf-Server")
@@ -124,7 +124,7 @@ struct LoginView: View {
             Image(systemName: "books.vertical.fill")
                 .font(.system(size: 48))
                 .foregroundStyle(.tint)
-            Text("ABS Client")
+            Text("Nook")
                 .font(.largeTitle).bold()
             Text("Verbinde dich mit deinem Audiobookshelf-Server")
                 .foregroundStyle(.secondary)