slight z-index refactor and attempt at organizing it

src/components/mobile_nav/mobile_nav.vue

@@ -86,6 +86,8 @@
 @import '../../_variables.scss';
 
 .MobileNav {
+  z-index: var(--ZI_navbar);
+
   .mobile-nav {
     display: grid;
     line-height: var(--navbar-height);
@@ -147,7 +149,7 @@
     transition-property: transform;
     transition-duration: 0.25s;
     transform: translateX(0);
-    z-index: 1001;
+    z-index: var(--ZI_navbar);
     -webkit-overflow-scrolling: touch;
 
     &.-closed {
@@ -160,7 +162,7 @@
     display: flex;
     align-items: center;
     justify-content: space-between;
-    z-index: 1;
+    z-index: calc(var(--ZI_navbar) + 100);
     width: 100%;
     height: 50px;
     line-height: 50px;