Fix export completion UI state

[Admin-AAA]

Summary

• Move background list/export completion handling through main-thread queues so Tkinter reliably updates the UI
• Add a reload-list button and clearer empty/loading states for chat/contact lists
• Stop the export progress indicator and restore buttons before showing success or error dialogs
• Add a focused unit test for successful export UI cleanup

Tests

• python3 -m py_compile wechat_exporter_gui.py tests/test_gui_export_state.py
• python3 -m unittest discover -s tests -p 'test_*.py'
• bash -n start_gui.sh

Note: local flake8 was not installed; repository CI installs and runs flake8.