Cannot drag & drop a sub-page on a pinned document shortcut

[mjeammet]

Bug Report

Problematic behavior
It seems the "pinned document" shortcut on the left panel doesn't allow drag & drop-ing a document to become another doc's subpage

Expected behavior/code
I would expect my document to become a sub-page (or display a warning) as it does when I drag & drop inside the central list component

Steps to Reproduce

1. Own/create a document
2. Be at least editor on a pinned document
3. Drag & drop your document onto the pinned one on left panel
4. You get all the visual cues but your document has not been added to the pinned document

Environment

• Docs version: prod as to July 10th = v3.4.0 ?
• Instance url: https://docs.numerique.gouv.fr/

Additional context/Screenshots
https://github.com/user-attachments/assets/4ba86f04-5dc7-4810-bf53-62931ea64d17

[virgile-dev]

Thanks @mjeammet!
Closing this in favor of #1156 (it's a bit more detailed)