fix: use i18next v4+ pluralization format and add sessionTitle fallback

src/components/Sidebar.jsx

@@ -583,7 +583,7 @@ function Sidebar({
                   <p className="text-sm text-muted-foreground mb-1">
                     {t('deleteConfirmation.confirmDelete')}{' '}
                     <span className="font-medium text-foreground">
-                      {sessionDeleteConfirmation.sessionTitle}
+                      {sessionDeleteConfirmation.sessionTitle || t('sessions.unnamed')}
                     </span>?
                   </p>
                   <p className="text-xs text-muted-foreground mt-3">

src/i18n/locales/en/sidebar.json

@@ -103,8 +103,8 @@
     "deleteProject": "Delete Project",
     "deleteSession": "Delete Session",
     "confirmDelete": "Are you sure you want to delete",
-    "sessionCount": "This project contains {{count}} conversation.",
-    "sessionCount_plural": "This project contains {{count}} conversations.",
+    "sessionCount_one": "This project contains {{count}} conversation.",
+    "sessionCount_other": "This project contains {{count}} conversations.",
     "allConversationsDeleted": "All conversations will be permanently deleted.",
     "cannotUndo": "This action cannot be undone."
   }

src/i18n/locales/zh-CN/sidebar.json

@@ -103,8 +103,8 @@
     "deleteProject": "删除项目",
     "deleteSession": "删除会话",
     "confirmDelete": "您确定要删除",
-    "sessionCount": "此项目包含 {{count}} 个对话。",
-    "sessionCount_plural": "此项目包含 {{count}} 个对话。",
+    "sessionCount_one": "此项目包含 {{count}} 个对话。",
+    "sessionCount_other": "此项目包含 {{count}} 个对话。",
     "allConversationsDeleted": "所有对话将被永久删除。",
     "cannotUndo": "此操作无法撤销。"
   }