[4.0] Mass mail subject prefix styling

[SharkyKZ]

Pull Request for Issue #29561.

Summary of Changes

Fixes subject field styling when prefix is enabled.

Testing Instructions

Go to com_users options. In Mass Mail Users tab fill Subject Prefix field.
Go to Users -> Mass Mail Users.

Expected result

Subject field looks OK.

Actual result

Subject field looks bad.

Documentation Changes Required

No.

[richard67]

This PR here is so clear that it even could be merged by review ... but I will do a real test.

[richard67]

I have tested this item ✅ successfully on 87e17d9

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[Quy]

I have tested this item ✅ successfully on 87e17d9

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/29572.}

[Quy]

RTC

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