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-sync/tests/sync.rs

@@ -61,6 +61,7 @@ impl Device {
         d.api
             .register(
                 SETUP_TOKEN,
+                None,
                 d.keyset.account_id,
                 &test_kdf(),
                 &AUTH,
@@ -410,6 +411,7 @@ async fn auth_errors() {
         .api
         .register(
             SETUP_TOKEN,
+            None,
             Uuid::new_v4(),
             &test_kdf(),
             &AUTH,