Add Darling Data favicon to web app

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

src/PlanViewer.Web/wwwroot/favicon.png

@@ -4,6 +4,7 @@
     <meta charset="utf-8" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>Free SQL Server Query Plan Analysis — Darling Data</title>
+    <link rel="icon" type="image/png" href="favicon.png" />
     <base href="/" />
     <link rel="preconnect" href="https://fonts.googleapis.com" />
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin />