feat: enhance new chat functionality with document mentions support

- Updated the new chat routes to include handling for mentioned document IDs, allowing users to reference specific documents in their chat.
- Modified the NewChatRequest schema to accommodate optional document IDs.
- Implemented document mention formatting in the chat streaming service for improved context.
- Enhanced the frontend to manage document mentions, including a new atom for state management and UI updates for document selection.
- Refactored the DocumentsDataTable component for better integration with the new mention functionality.

surfsense_web/atoms/chat/mentioned-documents.atom.ts

@@ -0,0 +1,10 @@
+"use client";
+
+import { atom } from "jotai";
+
+/**
+ * Atom to store the IDs of documents mentioned in the current chat composer.
+ * This is used to pass document context to the backend when sending a message.
+ */
+export const mentionedDocumentIdsAtom = atom<number[]>([]);
+