Add a passphrase-locked vault

A second notebook whose pages are encrypted with a key made from a passphrase plus the server key. It is unlocked into memory only (15 minutes idle, Lock, or sign-out), is not in search, lists or downloads of the ordinary notebook, and limits wrong guesses.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/vault/vault.go

@@ -8,8 +8,10 @@ import (
 	"crypto/aes"
 	"crypto/cipher"
 	"crypto/hkdf"
+	"crypto/pbkdf2"
 	"crypto/rand"
 	"crypto/sha256"
+	"encoding/hex"
 	"errors"
 	"fmt"
 )
@@ -50,6 +52,13 @@ func New(master, issuer, sub string) (*Cipher, error) {
 	return &Cipher{aead}, nil
 }
 
+// Passphrase stretches a passphrase into key material (hex) with PBKDF2-HMAC-SHA256, which is slow on
+// purpose so that guessing passphrases is expensive.
+func Passphrase(pass string, salt []byte, iterations int) (string, error) {
+	k, err := pbkdf2.Key(sha256.New, pass, salt, iterations, 32)
+	return hex.EncodeToString(k), err
+}
+
 // IsSealed says whether data is an encrypted file.
 func IsSealed(data []byte) bool { return bytes.HasPrefix(data, magic) }