feat(frontend): browser tab shows the open chat, empty state speaks the chosen language

frontend/src/lib/components/ChatHeader.svelte

@@ -139,6 +139,10 @@
   );
 </script>
 
+<svelte:head>
+  <title>{title} · Beavergram</title>
+</svelte:head>
+
 <header class="chat-header">
   <div class="back">
     <Button