Google Analytics: enable site-wide tracking with configuration

[Copilot]

Enables Google Analytics 4 tracking across the site using the minimal-mistakes-jekyll theme's custom head include pattern.

Changes:

• Added _includes/google-analytics.html with gtag.js implementation reading tracking ID from site.analytics.google.tracking_id
• Integrated via _includes/head/custom.html for site-wide coverage in the head section
• Configuration in _config.yml provides single source of truth for tracking ID

Implementation:

<!-- _includes/google-analytics.html -->
<script async src="https://www.googletagmanager.com/gtag/js?id={{ site.analytics.google.tracking_id }}"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag() { dataLayer.push(arguments); }
  gtag('js', new Date());
  gtag('config', '{{ site.analytics.google.tracking_id }}');
</script>

Uses the theme's standard override point (_includes/head/custom.html) rather than layout modification for maintainability.

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