[2.x] style: hide search clear button when collapsed in tablet mode

[huoxin233]

Fixes #4593

Changes proposed in this pull request:

In tablet mode, if the search state retains a value, the Input component still renders the .Input-clear button, causing it to visually overlap with the search prefix icon.

This PR adds a CSS rule using the sibling selector input:not(:focus) ~ .Input-clear within the existing tablet media query to hide the clear button when the input is collapsed.

Reviewers should focus on:

Screenshot

Before:

PixPin_2026-04-20_23-17-29.mp4

After:

PixPin_2026-04-21_01-18-44.mp4

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)