feat: make thread elements have the same space around them

[GretaD]

fixes #11316

Only worked on these points as added by Jan on the ticket:

• The mail heading & participants should be aligned to the content box.
• and the "Reply" button as well as the action buttons on the top right
• When scrolling the content, the container / top divider line vanishes and gets overlapped partly by the heading, making it look strange
• Now with the message list having more content, horizontal divider lines could make sense to have some nicer separation.
• Mails have different heights in the message list if there’s shorter ones – but all should have the same consistent height
• Whitespace is sometimes not the same across the board, for example the space left and right in the message list is much less (half?) than in the left navigation.

before

after

after(reply button aligned with the action menu)

Subject and recipient aligned with the recipient in the message header. plus added the line to split subject from the message

more space for the preview, but this is a bit tricky to add more space for the preview because the details, the unread and the action menu wouldnt allow it to go further.
before

after

[ChristophWurst]

Looks good otherwise

[nimishavijay]

Nice improvements :)
I am wondering if the changes in the last part of this comment can be incorporated as well? Could be as a separate PR too #11065 (comment) also cc @hamza221 as you are assigned to that issue

[GretaD]

Nice improvements :) I am wondering if the changes in the last part of this comment can be incorporated as well? Could be as a separate PR too #11065 (comment) also cc @hamza221 as you are assigned to that issue

they will be done for sure, but in a separate pr :)

[ChristophWurst]

IMO adjusting the width of the elements would have been preferable over this additional element

[GretaD]

i went for this because the border would lose the sticky position if i would do it like that. Either im doing something wrong with your proposal, or i dont know.