Fix web gui size

gui/index.html

@@ -7,5 +7,12 @@
             import init from "./pkg/inkopslista.js";
             init();
         </script>
+        <style>
+            #canvas {
+                /* Force Slint to occupy the entire viewport */
+                width: 100vw !important;
+                height: 100vh !important;
+            }
+        </style>
     </body>
 </html>