Desktop nav uses <button onclick> instead of <a> for navigation links

## Summary

The desktop navigation utility items (e.g. the "Sign Up" button) are rendered as `` elements with `onclick="window.location='...'"` or `onclick="window.open('...')"`. Buttons are semantically for actions, not navigation. This causes several problems:

- Screen readers announce these as "button" rather than "link", giving users incorrect expectations
- They cannot be middle-clicked to open in a new tab
- They do not support browser history navigation correctly
- `window.open()` can be blocked by popup blockers

## Affected location

`_includes/header.html:41–52`

```liquid
{% for item in site.data.navigation.utility %}

{{ item.label }}{% if item.external %} ↗{% endif %}


{% endfor %}
```

Note: the mobile menu already correctly uses `` for these same items — the desktop nav should match.

## Fix

Replace the `` pattern with `` elements styled to look like buttons, matching the mobile menu implementation:

```liquid
{% for item in site.data.navigation.utility %}

{{ item.label }}{% if item.external %} ↗{% endif %}

{% endfor %}
```

## WCAG criterion

4.1.2 Name, Role, Value (Level A), 2.1.1 Keyboard (Level A)

Start in _includes/header.html:41–52 and compare the desktop utility-item markup with the mobile menu implementation. Replace the desktop onclick navigation pattern with the requested anchor markup, preserving labels, styling, external-link behavior, and the indicator; done means navigation has link semantics and supports normal browser link interactions.

Written by the indexing model from the issue text.