[UI] Set a base `z-index` for the root content canvas.

This allows loading spinners, modals and other components with a real
fullscreen background to stretch over the required space, without being
covered by the navigator or other sibling components.

This also requires the collapsed navigator to have a 1px margin-right,
or its separation border won't be visible.

platypush/backend/http/webapp/src/components/Nav.vue

@@ -331,6 +331,7 @@ nav {
   &.collapsed {
     display: flex;
     flex-direction: column;
+    margin-right: 1px;
 
     @media screen and (min-width: $tablet) {
       width: 2.5em;

platypush/backend/http/webapp/src/views/Panel.vue

@@ -134,6 +134,7 @@ main {
     flex-grow: 100;
     background: $menu-panel-bg;
     overflow: auto;
+    z-index: 1;
 
     .panel {
       width: 100%;