Files: a move of something that isn't there is a 404, not a task that fails on arrival

The kernel answers EXDEV for two paths on different filesystems before it looks for the
source, so the cross-filesystem path was taken for a missing file.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/api/server.go

@@ -2088,6 +2088,12 @@ func (s *Server) filesMove(w http.ResponseWriter, r *http.Request) {
 		jsonErr(w, 400, "a folder can't be moved into itself")
 		return
 	}
+	// The kernel answers EXDEV for two paths on different filesystems before it looks for
+	// the source, so a missing file would otherwise become a task that fails on arrival.
+	if _, err := fsys.Stat(r.Context(), from); err != nil {
+		fsErr(w, err)
+		return
+	}
 	err = fsys.Rename(r.Context(), from, to)
 	if err == nil {
 		log.Printf("files: move %s -> %s (from %s)", from, to, clientIP(r))