Validate username and password in discovery credentials dialog

Show an error if either field is empty before attempting to connect.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

connect_dialog.py

@@ -384,6 +384,13 @@ class _CredentialsDialog:
         user = self._user_var.get().strip()
         password = self._pass_var.get()
 
+        if not user:
+            self._status_label.config(text="Username is required", foreground="#c00000")
+            return
+        if not password:
+            self._status_label.config(text="Password is required", foreground="#c00000")
+            return
+
         try:
             port_int = int(self._port)
         except ValueError: