dismissBackup

User chose "Start fresh" from the SigilStatus.BackupAvailable prompt: they acknowledge the cloud backup but want to proceed without restoring. Persist a flag so future launches don't keep nagging, and move to SigilStatus.None so the wallet panel unblocks and auto-bootstraps a fresh wallet.

The cloud backup itself is not deleted — user can still tap "restore from cloud" later in the SigilStatus.None sheet if they change their mind. The flag just controls the auto-prompt.