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.

crates/pdfs-core/src/control.rs

@@ -481,6 +481,11 @@ pub enum Request {
     /// [`Request::ListPendingOps`]) to `dest`, an absolute path that must not
     /// exist yet. The op stays queued. Replies with [`Response::Ok`].
     ExportPendingOp { id: i64, dest: String },
+    /// Drop a queued change (`id` from [`Request::ListPendingOps`]) and undo
+    /// it locally: a node made here goes, a node Drive has goes back to what
+    /// Drive has, along with every other change queued on it. Its content is
+    /// lost unless it was exported first. Replies with [`Response::Ok`].
+    DiscardPendingOp { id: i64 },
     /// List the `(sync-conflict …)` copies under My Files and in synced
     /// folders. Replies with
     /// [`Response::Conflicts`].
@@ -3354,6 +3359,10 @@ mod tests {
                 },
                 r#"{"ExportPendingOp":{"id":7,"dest":"/tmp/out.bin"}}"#,
             ),
+            (
+                Request::DiscardPendingOp { id: 7 },
+                r#"{"DiscardPendingOp":{"id":7}}"#,
+            ),
         ];
         for (request, wire) in cases {
             assert_eq!(serde_json::to_string(&request).unwrap(), wire);