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/Info-iOS.plist

@@ -5,7 +5,7 @@
 	<key>CFBundleDevelopmentRegion</key>
 	<string>$(DEVELOPMENT_LANGUAGE)</string>
 	<key>CFBundleDisplayName</key>
-	<string>ABS Client</string>
+	<string>Nook</string>
 	<key>CFBundleExecutable</key>
 	<string>$(EXECUTABLE_NAME)</string>
 	<key>CFBundleIdentifier</key>
@@ -13,7 +13,7 @@
 	<key>CFBundleInfoDictionaryVersion</key>
 	<string>6.0</string>
 	<key>CFBundleName</key>
-	<string>ABS Client</string>
+	<string>Nook</string>
 	<key>CFBundlePackageType</key>
 	<string>$(PRODUCT_BUNDLE_PACKAGE_TYPE)</string>
 	<key>CFBundleShortVersionString</key>