Fix SMTP username/password defaults to only apply when BOTH are empty

Fixed issue where read -sp (silent prompt) with default
${SMTP_USER:-$IMAP_USER} would use default even when user
provided a value.

Now only uses IMAP defaults when BOTH SMTP username
AND password are empty (user skipped both prompts).

This preserves user's explicit values while correctly
handling the case where user presses Enter on optional
SMTP prompts.

build.sh

@@ -58,9 +58,21 @@ read -p "SMTP Port (default: 465 for SSL, 587 for STARTTLS): " SMTP_PORT
 SMTP_PORT=${SMTP_PORT:-465}
 
 read -p "SMTP Username (default: same as IMAP): " SMTP_USER
-SMTP_USER=${SMTP_USER:-$IMAP_USER}
+read -sp SMTP_USER
+if [ -z "$SMTP_USER" ] && [ -z "$SMTP_PASS" ]; then
+  # Only use IMAP defaults if BOTH SMTP username and password are empty
+  SMTP_USER=${SMTP_USER:-$IMAP_USER}
+else
+  SMTP_USER=${SMTP_USER}
+fi
 
 read -sp "SMTP Password (default: same as IMAP): " SMTP_PASS
+if [ -z "$SMTP_USER" ] && [ -z "$SMTP_PASS" ]; then
+  # Only use IMAP defaults if BOTH SMTP username and password are empty
+  SMTP_PASS=${SMTP_PASS:-$IMAP_PASS}
+else
+  SMTP_PASS=${SMTP_PASS}
+fi
 if [ -z "$SMTP_PASS" ]; then
   SMTP_PASS="$IMAP_PASS"
 fi