niri: float the Thunar file-operation progress dialog

The progress window for copy/move/delete operations is small and
transient — floating it stops it from taking over a column.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

niri/config.kdl

@@ -148,3 +148,9 @@ window-rule {
         off
     }
 }
+
+// Thunar file-operation progress dialog
+window-rule {
+    match app-id="^thunar$" title="^File Operation Progress$"
+    open-floating true
+}