feat(core,fuse,cli,gui): discard a queued change and undo it here

A change Drive will not take could be exported or retried, never let go
of. The Sync page's Discard button (with a confirmation) and
`pdfs sync discard <id>` drop it: a node made here goes, as if deleted
before it was sent; a node Drive has goes back to what Drive has, under
the same inode, with every change queued on it, while what is queued
below it stays. A change on the wire or an open file is refused.

docs/TROUBLESHOOTING.md

@@ -128,7 +128,13 @@ pdfs sync retry 42        # one operation
 ```
 
 The app shows the same list on **Sync → Overview**. An operation that keeps failing shows its
-error there.
+error there. A change Proton Drive will not take can be dropped instead, which undoes it here;
+save a file's content first if you want to keep it:
+
+```bash
+pdfs sync export 42 ~/Desktop/
+pdfs sync discard 42
+```
 
 **A file in a synced folder does not upload** — check that: