feat: show background color for direct post to reduce mistake

[shuuji3]

resolves #2723

Screenshots

Also, it properly expands the background when it has many lines:

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[patak-dev]

Ah, this was such an important missing piece 🙌🏼

[patak-dev]

Trying to see why the elkdev user doesn't work locally for me 🤔
In the meantime, after using this for a bit, the styles in this PR are good for the main editor. I was thinking if we should have a bit of gradient to avoid the feeling of going to send a bunch of empty lines.

There is also some missing bottom padding for the reply editor in threads

And the same missing bottom padding in both the main and thread editors when it reached the expansion point

[shuuji3]

Good point.👍🏻 If the message is a single line, I think the background should be the same height as the posted message. And, the bottom margin/padding seems to be less than the top one. I'll adjust this unbalanced padding/margin.