willchen96 stops documents from wandering into the wrong matter

Mike lets you organise documents into folders inside a project. Until now, the actions that rearrange that structure - moving a folder, deleting one, or reassigning a document - didn't confirm the folder you named was really part of the project you named. If folder IDs happened to collide across projects, a request could quietly act on the wrong one, and a document could end up detached from the wrong matter.

The fix is small but pointed: every one of those actions now verifies the folder lives in the referenced project first, and refuses cleanly if it doesn't. A request pointing at a folder in someone else's project, or at nothing at all, gets turned away instead of silently proceeding on bad data.