Add validation to prevent empty usernames/passwords

Build script now validates that IMAP username and password
are not empty before proceeding with the build.

This prevents the binary from being built with empty
credentials which would cause connection errors like:
  "dial tcp :143" (empty hostname)

build.sh

@@ -38,7 +38,17 @@ read -p "IMAP Port (default: 993 for SSL, 143 for STARTTLS): " IMAP_PORT
 IMAP_PORT=${IMAP_PORT:-993}
 
 read -p "IMAP Username: " IMAP_USER
+if [ -z "$IMAP_USER" ]; then
+  echo "❌ Error: IMAP username cannot be empty"
+  exit 1
+fi
+
 read -sp "IMAP Password: " IMAP_PASS
+if [ -z "$IMAP_PASS" ]; then
+  echo "❌ Error: IMAP password cannot be empty"
+  exit 1
+fi
+
 echo ""
 
 read -p "SMTP Server hostname (default: teamworkapps.com): " SMTP_SERVER
@@ -51,12 +61,15 @@ read -p "SMTP Username (default: same as IMAP): " SMTP_USER
 SMTP_USER=${SMTP_USER:-$IMAP_USER}
 
 read -sp "SMTP Password (default: same as IMAP): " SMTP_PASS
-echo ""
-
 if [ -z "$SMTP_PASS" ]; then
   SMTP_PASS="$IMAP_PASS"
 fi
 
+if [ -z "$SMTP_PASS" ]; then
+  echo "❌ Error: SMTP password cannot be empty (IMAP password also empty)"
+  exit 1
+fi
+
 read -p "Use SSL/TLS? (Y/n, default: Y): " USE_SSL
 USE_SSL=${USE_SSL:-Y}