AO3-6806 Fix first comment layout on short admin posts

[noahbravo]

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Issue

https://otwarchive.atlassian.net/browse/AO3-6806

Purpose

Fixes a layout issue where the first comment on a short admin post had an oversized byline and a misplaced icon when the comment form was not present.

Root cause:

• The #dashboard sidebar has float: left at page level, and comment pseudo-elements have clear: both. When an admin post is shorter than the sidebar, the first comment's clear: both catches the active sidebar float and forces the comment to expand until the float clears.
• When the comment form is present, its height pushes the comments below the sidebar and the issue does not occur.

Changes made:

• Added a scoped clear: none rule under .news.admin.home in 15-group-comments.css to prevent comment pseudo-elements from interacting with the sidebar float. Only affects admin post views. Works and chapters are unaffected.
• Replaced slideDown()/slideUp() with show()/hide() on admin posts in show_comments.js.erb and hide_comments.js.erb. The slide animation creates a temporary BFC that triggers a layout reflow on completion, which was hidden before the CSS fix but became noticeable after. Works and chapters are unaffected.

Testing Instructions

Follow the reproduction steps in AO3-6806:
https://otwarchive.atlassian.net/browse/AO3-6806

Expected behavior:

• The first comment on a short admin post should render normally, with the byline and icon displayed at the correct height.
• The rest of the comments should be unaffected.
• Comments on works and chapters should still animate with slideDown/slideUp as before.
• No visual difference on admin posts where the comment form is present.

Automated tests:

• No automated tests were added for this change, as the bug is a visual/layout issue dependent on relative content length.

Credit

Name: Noah Bravo
Pronouns: they/them