[web-client] simple theme: dynamic vertical height

On mobile devices with browser chrome (UI) that recedes on scroll,
'vh' includes the height covered by chrome, which 'dvh' does not.
By using 'dvh' the footer is immediately visible on the homepage
without scrollbar on mobile devices.

client/simple/src/less/style.less

@@ -80,6 +80,12 @@ body {
   margin: 0;
 }
 
+@supports (height: 100dvh) {
+  body {
+    height: 100dvh;
+  }
+}
+
 main {
   width: 100%;
   margin-bottom: 2rem;