fix: composer title positioning incorrect with custom header height

[davwheat]

Changes proposed in this pull request:

With a custom header height specified, the mobile composer slips up, with some elements appearing behind the header as if it isn't bigger, and the title isn't centred correctly.

This PR fixes both of these issues, as seen in the screenshots below.

Reviewers should focus on:

N/A

Screenshot

Before and after screenshots:

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.