Fix #359: web drops land in the document they were dropped on

The web reads dropped files asynchronously, and the place inbox only kept
the drop point, so a file dropped on document A was placed into whatever
document was active when its bytes arrived. A drop on a canvas now carries
place::DropAt (the document's uid, the point, embed), and drop_files makes
that document active again before placing; when it has closed meanwhile,
nothing is placed and the status bar says so. The native drop path is
synchronous and goes through the same code, so it is unchanged in effect.

Test: a drop pending while another document opens lands in the first one
at the drop point; one whose document closes is not placed anywhere.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

apps/vectorcraft-web/src/web.rs

@@ -108,8 +108,8 @@ impl eframe::App for WebShell {
                     match f.bytes_async().await {
                         Ok(bytes) => {
                             match target {
-                                DropTarget::Place { at, embed } => {
-                                    place_inbox.lock().unwrap_or_else(|e| e.into_inner()).push(PlaceArrival { name, bytes, drop: Some((at, embed)) })
+                                DropTarget::Place(d) => {
+                                    place_inbox.lock().unwrap_or_else(|e| e.into_inner()).push(PlaceArrival { name, bytes, drop: Some(d) })
                                 }
                                 DropTarget::Open => inbox.lock().unwrap_or_else(|e| e.into_inner()).push((name, bytes)),
                             }