Add favicon to web application

Added SVG favicon based on KeepItGoing app icon (white checkmark on
blue #3B82F6 background). Favicon displays in browser tabs and bookmarks.

Changes:
- Created static/favicons/favicon.svg with app icon design
- Updated templates/base.html with favicon link tags

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

templates/base.html

@@ -5,6 +5,11 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{% block title %}KeepItGoing{% endblock %}</title>
+
+    <!-- Favicon -->
+    <link rel="icon" type="image/svg+xml" href="{% static 'favicons/favicon.svg' %}">
+    <link rel="alternate icon" href="{% static 'favicons/favicon.svg' %}" type="image/svg+xml">
+
     <link rel="stylesheet" href="{% static 'css/app.css' %}">
     {% block extra_css %}{% endblock %}
 </head>