[Feature Request]: allow environmental variable control checkbox "Use HTML format in mail body" in "Mail" transform

### What would you like to happen?

### What would you like to happen?

Follow-up of https://github.com/apache/hop/issues/5628.

For "Mail transform" we would like to be able to control the "Use HTML format in mail body" checkbox via an environmental variable:

Documentation would include valid values for this variable, and the logic that is applied when the variable is empty/unset, or contains invalid values.
Similary, what would happen when the checkbox conflicts with the value of the variable.
Optimally, the checkbox would be greyed out when the config variable is used?

### Issue Priority

Priority: 3

### Issue Component

Component: Hop Gui, Component: Transforms

Start with the Mail transform in the Hop GUI and review follow-up issue #5628 to trace how the “Use HTML format in mail body” checkbox is handled. Define the environment variable’s valid and unset values, conflict behavior, and whether the checkbox is disabled; document those rules and verify the resulting UI and transform behavior.

Written by the indexing model from the issue text.