Add time-of-day greeting variable

Problem
When composing messages, users often lack the customer's name or want something more formal than a generic greeting. There's no way to automatically adjust greetings based on time of day (morning, afternoon, evening).

Request
Add a dynamic variable for time-of-day greetings (e.g., "Good morning", "Good afternoon", "Good evening") that automatically displays the appropriate greeting based on the current time when the message is sent.