Fix web gui scaling on mobile

gui/index.html

@@ -1,4 +1,7 @@
 <html>
+    <head>
+        <meta name="viewport" content="width=device-width, height=device-height, initial-scale=1">
+    </head>
     <body>
         <!-- canvas required by the Slint runtime -->
         <canvas id="canvas"></canvas>