feat: support encrypted subscription decryption (AES-128-CBC)

- Add subscription_decrypt.dart: isSubscriptionEncrypted, tryDecryptSubscription, decryptSubscriptionResponse, SubscriptionEncryptedException (ref #1346) - server_manager: subscriptionPassword param for addRemoteConfig/loadFrom, persist passwords for auto-update, pass to tryConvert (upstream to add 8th arg) - Add/edit profile screens: optional subscription password field - pubspec: add encrypt 5.0.3 Closes #1346