Addresses two issues preventing configuration backup/restore:
1. Export passphrase validation mismatch: UI only validated 12+ char
requirement when using custom passphrase, but backend always enforced
it. Users with shorter login passwords saw unexplained failures.
- Frontend now validates all passphrases meet 12-char minimum
- Clear error message suggests custom passphrase if login password too short
2. Import data parsing failed silently: Frontend sent `exportData.data`
which was undefined for legacy/CLI backups (raw base64 strings).
Backend rejected these with no logs.
- Frontend now handles both formats: {status, data} and raw strings
- Backend logs validation failures for easier troubleshooting
Related to #646 where user reported "error after entering password" with
no container logs. These changes ensure proper validation feedback and
make the backup system resilient to different export formats.
7ed9203e4b