Allow calls to display edge-to-edge on mobile

By adding viewport-fit=cover to the <meta name="viewport"> header, the page now requests to be displayed edge-to-edge across the entire screen. This gives us control over what we display around camera cut-outs and system navigation UI, if the user agent supports it. I then adjusted the styles of various UI elements to ensure that they still lie within the screen's safe area.

src/AppBar.module.css

@@ -1,10 +1,9 @@
 .bar {
-  block-size: 64px;
   flex-shrink: 0;
 }
 
 .bar > header {
-  position: absolute;
+  position: sticky;
   inset-inline: 0;
   inset-block-start: 0;
   block-size: 64px;