isWatchOnly abstract method
Is this a WATCH-ONLY wallet (#397 §3.7 D4/D5)? The reference UI keys its
chrome on this: a "Watch-only" header badge, hidden Send/Shield/Swap
affordances, and the Security screen's no-backup ("Export viewing key")
arm. The typed WalletErrorKind.watchOnly refusals stand at the SDK
regardless, so a host ignoring this only ever sees honest errors — never
a wrong behavior. Cheap, local, no network, no key material.
Implementation
Future<bool> isWatchOnly();