Opening a /f/<fileid> link to an inaccessible file should show a clear error, not the root folder

Describe the feature you'd like to request

When a logged-in user opens an internal link (https://<host>/index.php/f/<fileid>) to a file or folder not shared with them, the Files app lands on the root folder. A toast "File could not be found" appears top-right for a few seconds, with only a thin red left border, then disappears. New users don't notice it and believe the root folder is the content their colleague sent.

Describe the solution you'd like

Land on a page, or show a persistent banner in the file list, saying that the link cannot be opened with the current account, with a hint to ask the sender for access. Whether the file exists need not be revealed.

Describe alternatives you've considered

Keep the toast but make it persistent and styled as a warning. Keep the target fileid in the URL so the user can see the link was not resolved.

Additional context

Nextcloud 33.0.6. Reproduced with two colleagues on the same instance: the link worked once the sender shared the parent folder.

Start in the Files app entry path for index.php/f/ and trace how an inaccessible file or folder falls back to the root folder and displays the toast. Done means the unresolved link shows a clear, persistent access error with an access hint, while not revealing whether the target exists; verify both file and folder links.