Describe power levels <0 as "muted" in event text

src/backend/nio_callbacks.py

@@ -367,7 +367,8 @@ class NioCallbacks:
             return (
                 f"Admin ({level})"     if level == 100 else
                 f"Moderator ({level})" if level >= 50 else
-                f"User ({level})"
+                f"User ({level})"      if level >= 0 else
+                f"Muted ({level})"
             )
 
         def format_defaults_dict(