bugfix: notification kist display on small screen

src/styles/base/header.scss

@@ -7,12 +7,13 @@
   color: var(--color-black);
   box-shadow: 0 2px 4px rgba(0, 0, 0, 0.09);
   flex-shrink: 0;
-  position: relative;
-  z-index: 1;
+  position: fixed;
+  z-index: 999;
 
   @include breakpoint(lg) {
     max-height: 83px;
     height: 83px;
+    position: relative;
   }
 
   &.is-spaced {

src/styles/components/chat.scss

@@ -1,6 +1,6 @@
 .chat-view {
   padding: 0;
-  height: calc(100vh - 50px - 56px); /* Header's and subheader's height on mobile */
+  height: calc(100vh);
   position: relative;
   overflow: hidden;