UX: composer fixes (#23334)

* UX: fix disappearing separator

* UX: slightly smaller composer btns on non-mobile

plugins/chat/assets/stylesheets/common/chat-composer-button.scss

@@ -1,9 +1,12 @@
 .chat-composer-button {
+  @include breakpoint(mobile-large) {
+    width: 50px;
+  }
   box-sizing: border-box;
-  width: 50px;
   border: 0;
   height: 50px;
   background: none;
+  padding-inline: 0.75rem;
 
   .is-disabled & {
     cursor: not-allowed;

plugins/chat/assets/stylesheets/common/chat-composer.scss

@@ -29,6 +29,10 @@
     align-self: flex-end;
   }
 
+  .chat-composer-separator {
+    flex-shrink: 0;
+  }
+
   &__outer-container {
     display: flex;
     align-items: center;