Migrate legacy ui.disableLoadingPhrases to ui.enableLoadingPhrases

[acoliver]

Summary\nSettings migration currently handles accessibility.disableLoadingPhrases and ui.accessibility.disableLoadingPhrases, but legacy configs can also contain ui.disableLoadingPhrases.\n\nExpected\nWhen ui.disableLoadingPhrases is present, migration should write ui.enableLoadingPhrases with inverted polarity and remove the deprecated key when cleanup is enabled.\n\nAcceptance criteria\n- Add migration support for ui.disableLoadingPhrases\n- Keep compatibility behavior when both old and new keys exist\n- Add or update tests covering the ui-level key path