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

frontend/src/lib/i18n/messages/common.ts

@@ -6,4 +6,8 @@ export const common = {
   "common.close": { en: "Close", ru: "Закрыть" },
   "common.delete": { en: "Delete", ru: "Удалить" },
   "common.save": { en: "Save", ru: "Сохранить" },
+  "common.selectChat": {
+    en: "Select a chat to open its archive",
+    ru: "Выберите чат, чтобы открыть архив",
+  },
 } satisfies Messages;

frontend/src/routes/app/+page.svelte

@@ -1,5 +1,9 @@
+<script lang="ts">
+  import { t } from "#lib/i18n/locale.svelte.js";
+</script>
+
 <div class="placeholder">
-  <span class="pill">Select a chat to view its archive</span>
+  <span class="pill">{t("common.selectChat")}</span>
 </div>
 
 <style lang="scss">