- validate legacy-decoded wallet passwords are real generateKey() output
(696 chars, base64 of 512-byte key + 8-byte iv) so a wrong AES key that
unpads to printable bytes can't be re-saved as a bogus password
- test/legacy_decode_test.dart: proves 1.0.0->1.0.2 recovers PIN+password
and 16k wrong keys yield zero false-accepts