On some Android builds the mobile_scanner native plugin is not registered,
causing a MissingPluginException when the send/receive screens try to open
the QR scanner. Add a pre-flight _initScanner() method that starts and
immediately stops a temporary MobileScannerController in a try/catch; any
exception (including MissingPluginException) sets _scannerFailed=true and
the UI falls back to the existing copy-paste text-input flow instead of
leaving the user stuck with a blank camera view.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>