Write encrypted money-seed file with 0600 permissions on Unix

WalletSeed::init_file now creates wallet.seed owner-only (0600) via
OpenOptionsExt, mirroring the Nostr nsec in src/nostr/identity.rs, so
other local users cannot read it for an offline password grind. Uses
#[cfg(unix)] so non-Unix still compiles. KDF unchanged.

src/wallet/seed.rs

@@ -18,7 +18,6 @@ use grin_wallet_impls::Error;
 use rand::{Rng, rng};
 use serde_derive::{Deserialize, Serialize};
 use serde_json;
-use std::fs::File;
 use std::io::Write;
 
 use ring::aead;
@@ -48,9 +47,32 @@ impl WalletSeed {
 		let seed = WalletSeed::from_mnemonic(recovery_phrase)?;
 		let enc_seed = EncryptedWalletSeed::from_seed(&seed, password)?;
 		let enc_seed_json = serde_json::to_string_pretty(&enc_seed).map_err(|_| Error::Format)?;
-		let mut file = File::create(seed_file_path).map_err(|_| Error::IO)?;
-		file.write_all(&enc_seed_json.as_bytes())
-			.map_err(|_| Error::IO)?;
+		// The encrypted seed is the wallet's highest-value secret. Write it
+		// owner-only (0600) on Unix, mirroring the Nostr nsec (see
+		// src/nostr/identity.rs write_private), so other local users can't read
+		// it and attempt an offline password grind. Non-Unix keeps default perms.
+		#[cfg(unix)]
+		{
+			use std::os::unix::fs::{OpenOptionsExt, PermissionsExt};
+			let mut file = std::fs::OpenOptions::new()
+				.write(true)
+				.create(true)
+				.truncate(true)
+				.mode(0o600)
+				.open(seed_file_path)
+				.map_err(|_| Error::IO)?;
+			file.write_all(&enc_seed_json.as_bytes())
+				.map_err(|_| Error::IO)?;
+			// Tighten perms if the file already existed with a looser mode.
+			let _ =
+				std::fs::set_permissions(seed_file_path, std::fs::Permissions::from_mode(0o600));
+		}
+		#[cfg(not(unix))]
+		{
+			let mut file = std::fs::File::create(seed_file_path).map_err(|_| Error::IO)?;
+			file.write_all(&enc_seed_json.as_bytes())
+				.map_err(|_| Error::IO)?;
+		}
 		Ok(seed)
 	}
 }