Use company_site favicon for the tab icon (#10)

## Summary
- Copy company_site square `favicon.png` (gear/infinity mark) into `site/core/static/core/img/`
- Point `<link rel="icon">` at it instead of the wide wordmark `logo.png`

Closes #9

## Test plan
- [ ] Load the landing page and confirm the browser tab uses the square company mark, not the full wordmark
- [ ] Confirm `logo.png` is unchanged (still available for in-page branding)

Reviewed-on: #10

site/core/templates/core/base.html

@@ -11,7 +11,7 @@
   <link href="https://fonts.googleapis.com/css2?family=Inter:wght@300;400;600;800&display=swap" rel="stylesheet">
   <link href="{% static 'core/css/style.css' %}" rel="stylesheet">
   <link href="{% static 'core/css/overrides.css' %}" rel="stylesheet">
-  <link rel="icon" type="image/png" href="{% static 'core/img/logo.png' %}">
+  <link rel="icon" type="image/png" href="{% static 'core/img/favicon.png' %}">
 </head>
 <body{% if debug %} class="has-debug-banner"{% endif %}>
   {% if debug %}