fix: styles on .user-prompt-buttons

[yanCode]

As in this snapshot, if you create empty prompts without editing the conent, nothing is wrong see A, but as long as you have long contents, well, the button groups starts displaying incorrectly, see B.

The reason is,

.user-prompt-header {
        max-width: calc(100% - 100px);
}

it only leaves 100px for the 3 buttons with full prompt contents. I don't think to reduce the max width of the content is a good idea, because we also need to display the content in mobile devices.

so the only way i can do is to slim the buttons, originally, each button has a padding of 10px, which takes up 20 px space, so i reduced it to padding: 7px;, not only that, i see the gap between two buttons from 5px to 2px.
by the way, I recommend using css properties of column-gap or row-gap in both flex and grid layouts , which is sightly better than

&:not(:last-child) {
            margin-right: 5px;
          }

also I commented out height: 100%;, if you need, you may change it back.

[Yidadaa]

LGTM, this style bug has been existing for a long time, thx.