break: empty Message.Mode when no pomodoro ongoing

- change "none" to an empty string for the mode field
- update README to reflect the new mode representation
- ensure consistency across session end/reset and welcome messages

🤖

internal/shared/state.go

@@ -5,7 +5,7 @@ import (
 )
 
 var Message = models.ServerMessage{
-	Mode:         "none",
+	Mode:         "",
 	Session:      0,
 	TotalSession: 0,
 	TimeLeft:     0,