fix(a11y): add cancel close button to SiteManagerDialog

[Orinks]

Summary

• add a dedicated Close button to SiteManagerDialog with id wx.ID_CANCEL
• keep existing Escape key handling (EVT_CHAR_HOOK) for keyboard dismissal
• extend dialog tests to verify close button exists and uses wx.ID_CANCEL

Testing

• ruff check src/portkeydrop/dialogs/site_manager.py tests/test_site_manager_dialog.py
• python -m pytest tests/test_site_manager_dialog.py -q
• python -m pytest -q

Closes #34