fix panel headers on mobile

src/components/chat/chat.scss

@@ -47,7 +47,7 @@
   }
 
   .chat-view-heading {
-    grid-template-columns: auto 1fr;
+    grid-template-columns: auto minmax(50%, 1fr);
   }
 
   .go-back-button {

src/panel.scss

@@ -41,7 +41,7 @@
   box-sizing: border-box;
   display: grid;
   grid-auto-flow: column;
-  grid-template-columns: 1fr;
+  grid-template-columns: minmax(50%, 1fr);
   grid-auto-columns: auto;
   grid-column-gap: 0.5em;
   flex: none;