[2.x] Inconsistent Return button height when navigating

### Current Behavior

The height of the return button is not kept the same when navigating from discussion to profile page. (Can be seen in the screenshot, reproducible on discuss)
The button height seems to have some relation with the current screen width also.

### Steps to Reproduce

1. Open https://discuss.flarum.org (the main page)
2. Then open into a discussion
3. See the return button on the top left and navigate to the profile page
4. The return button on the profile page is slightly shorter

Note that if open the discussion page directly then you wont see a difference in button height when navigating as the height is already the same

### Expected Behavior

The height of the button should keep constant throughout the navigation.

### Screenshots

Navigating from discussion to the profile page:

Screen width changes the button height:

### Environment

- Flarum version: 2.0.0-rc.6
- Website URL: https://discuss.flarum.org
- Webserver: [e.g. apache, nginx]
- Hosting environment: [e.g. shared, vps]
- PHP version: x.y.z
- Database: [e.g. MySQL 8.4.11, MariaDB 10.11.2, PostgreSQL 16.1, SQLite 3.45]
- Browser: [e.g. chrome 67, safari 11]

### Output of `php flarum info`

```
Output of "php flarum info", run this in terminal in your Flarum directory.
```

### Possible Solution

_No response_

### Additional Context

_No response_

Reproduce the issue on discuss.flarum.org by navigating from the main page into a discussion, then to a profile, and compare the return button at different screen widths. Trace the return-button rendering and styling used by the discussion and profile views; done means its height remains consistent throughout navigation and across the reported widths.

Written by the indexing model from the issue text.