Fix KDF metadata handling and headless password prompts

src/tests/test_seed_migration.py

@@ -1,4 +1,6 @@
 import sys
+import json
+import base64
 from pathlib import Path
 from cryptography.fernet import Fernet
 
@@ -28,4 +30,5 @@ def test_parent_seed_migrates_from_fernet(tmp_path: Path) -> None:
 
     assert new_file.exists()
     assert new_file.read_bytes() != encrypted
-    assert new_file.read_bytes().startswith(b"V2:")
+    payload = json.loads(new_file.read_text())
+    assert base64.b64decode(payload["ct"]).startswith(b"V2:")