refactor(sidebar): streamline conversation list with completed filter and expand toggle

Replace flat/grouped view toggle with a show-completed filter dropdown,
add expand/collapse-all control, extract status icon component, and
simplify the sidebar header.

src/i18n/messages/fr.json

@@ -834,8 +834,12 @@
       "statsLabel": "{folders} dossiers · {convos} conversations",
       "openFolder": "Ouvrir le dossier",
       "searchPlaceholder": "Rechercher des conversations...",
-      "viewFlat": "Vue à plat",
-      "viewGrouped": "Grouper par dossier",
+      "showCompleted": "Afficher les conversations terminées",
+      "moreOptions": "Plus d'options",
+      "statusRunningBadge": "En cours",
+      "statusFailedBadge": "Échec",
+      "conversationCountUnit": "{count, plural, one {# conversation} other {# conversations}}",
+      "emptyFolderHint": "Aucune conversation",
       "noMatchingConversations": "Aucune conversation correspondante",
       "removeFolderConfirmTitle": "Retirer le dossier de l'espace de travail ?",
       "removeFolderConfirmDescription": "Retirer \"{name}\" de l'espace de travail ? Les onglets et terminaux associés seront fermés.",