[2.x] fix: correct mobile layout and alignment of avatar in edited posts

[huoxin233]

Fixes #0000

Changes proposed in this pull request:

When a user edits an existing post on a mobile device (@media @phone), the layout is broken:

1. Duplicate Avatars: The user's avatar is displayed twice (once in the left sidebar space, and once inline in the header).
2. Misaligned Default Avatars: If the user has a default letter avatar, the letter is pushed to the top-left corner instead of being perfectly centered.
   (reproducible on discuss.flarum.org)

This PR add a CSS rule to

1. Hide the .Post-header .Post-avatar whenever .CommentPost.editing is active in mobile view.
2. Changed .Post-side .Post-avatar from display: block; to display: inline-flex; inside the editing block.

Misalignment of badges should be fix in #4582

Reviewers should focus on:

Screenshot

Before:

After:

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)