Fix Qt cleanup warning on dialog close

Add closeEvent that waits for thread pool workers to finish
before closing, preventing "shared QObject deleted directly" warning.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ramble/dialog.py

@@ -428,6 +428,11 @@ class RambleDialog(QDialog):
         self.result = RambleResult(summary=summary, fields=out_fields)
         self.accept()
 
+    def closeEvent(self, event):
+        """Wait for background workers to finish before closing."""
+        self.thread_pool.waitForDone(1000)  # Wait up to 1 second
+        super().closeEvent(event)
+
 
 def open_ramble_dialog(
     *,