fix: add apple-touch-icon to website for link preview icon

Messaging apps (WhatsApp, iMessage, etc.) use the apple-touch-icon to
display a site icon in link previews. Copy the existing icon from the
app and add the link tag to the landing page.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

website/index.html

@@ -16,6 +16,7 @@
 	<meta name="twitter:image" content="https://crumbs.bretzel.app/assets/og-image.png">
 	<link rel="icon" type="image/png" sizes="96x96" href="assets/favicon-96x96.png">
 	<link rel="icon" type="image/x-icon" href="assets/favicon.ico">
+	<link rel="apple-touch-icon" sizes="180x180" href="assets/apple-touch-icon.png">
 	<script defer src="https://umami.bretzel.app/script.js" data-website-id="5969ad34-0b49-4af4-bddf-b026358a97af"></script>
 	<link rel="stylesheet" href="styles.css">
 </head>