Multi-user: email sign-in, invite/open registration, admin calls

- kv-proto: email on setup, AccountInfo.email/is_admin, ServerInfo.registration, admin DTOs.
- kv-sync: prelogin by email or account id (setup QR / Emergency Kit), register with email,
  set_email, /v1/admin/* client calls.
- kv-ffi: register_account(code, email), login(.., email), account_details, set_account_email,
  admin_create_invite. kv-wasm: the same plus the full admin API for the web vault.
- Live test: two users on one server stay isolated; a new device signs in by email.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/kv-ffi/src/onboarding.rs

@@ -155,7 +155,7 @@ impl VaultCore {
         }
         self.configure_server(p.server_url.clone(), p.spki_pins.clone())?;
         let sk = SecretKey::parse(&p.secret_key)?;
-        self.login_inner(master_password, sk, None, p.ticket.clone(), Some(p.account_id)).await
+        self.login_inner(master_password, sk, None, p.ticket.clone(), Some(p.account_id), None).await
     }
 
     /// Joining device (no camera): opens a rendezvous on `server_url` and returns the QR to show.