Add SVG favicon matching logo design

Creates a scalable favicon with the same design as the header logo:
dark background with coral-colored circle, grid lines, and center dot.

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

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

frontend/index.html

@@ -11,6 +11,9 @@
     <meta name="author" content="SchoolCompare">
     <meta name="robots" content="index, follow">
 
+    <!-- Favicon -->
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg">
+
     <!-- Canonical -->
     <link rel="canonical" href="https://schoolcompare.co.uk/">