Setup wizard currently overwrites existing vaults silently. Adds a
mode picker (create new / attach this device), a vault-presence probe
after the connection test, and a Step 3b that verifies passphrase +
reference image by decrypting the manifest before registering a new
device key. Refuses destructive overwrite from the GUI; users wanting
a clean slate must delete the repo via their host's web UI.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
