The stash export was plaintext JSON, so a leaked/shared file exposed the whole
gear list. Export now wraps the payload in a version-2 JSON envelope:
- password mode: scrypt (NIP-49 params, 2^16/r8/p1) -> AES-256-GCM, with the
header (mode+cipher+KDF) bound as AAD so it can't be downgraded.
- nip44 mode: NIP-44-encrypted to your own key (no password to remember; needs
the matching key to open).
- plaintext v1 exports still import unchanged.
New src/lib/stashBackup.ts (verified round-trip, wrong-password, tamper and
KDF-downgrade rejection) + StashBackupExportDialog/StashBackupUnlockDialog.
The import picker only prompts for a password on password-encrypted files.
Password mode needs no signer (Amber-friendly); 'My key' is hidden unless the
signer supports NIP-44.
