Add conversation deletion functionality and update HistoryPage

- Implement a new API endpoint for deleting conversations in the backend.
- Enhance the HistoryPage component to include a dropdown menu for conversation actions, allowing users to delete conversations.
- Update the pagination logic to handle conversation removal and improve user experience during deletion.
- Adjust the loading state and error handling for the delete operation, ensuring smooth interaction.

frontend/src/lib/api.ts

@@ -249,6 +249,8 @@ export const conversationsApi = {
   },
   get: (id: string) =>
     request<ConversationDetail>(`/api/conversations/${id}`),
+  remove: (id: string) =>
+    request<{ ok: boolean }>(`/api/conversations/${id}`, { method: "DELETE" }),
 };
 
 // ---------- 工具 ----------