Add a unique icon so users can easily identify the site in their browser

Adds a favicon.svg to the client public directory and references it in index.html.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 91318273-c764-4fd4-be04-bdc12c38af32
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/0715008c-7244-40f4-befc-26c014852236/9d9e1107-c416-4124-a939-a91c612c56a1.jpg

client/index.html

@@ -5,6 +5,8 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1" />
     <title>RegisFlow</title>
     <meta name="description" content="Application de gestion des ventes de feux d'artifice conforme à la réglementation française" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <link rel="alternate icon" href="/favicon.ico" />
   </head>
   <body>
     <div id="root"></div>