* fix: Allow locale preview on onboarding preferences page
When a user selects a different language on /onboarding/preferences,
the page now immediately displays in the selected language. This is
achieved by checking for a valid locale URL parameter before falling
back to the family's saved locale setting.
* fix: Harden locale param handling and restore locale in tests
- Add type check to ensure params[:locale] is a String before calling
.to_sym, preventing 500 errors from array/hash injection attacks
- Add teardown to tests to restore original locale, preventing test
pollution
* fix: Reload family in teardown to handle update_column
* fix: Remove edge case test that used update_column with nil locale
* fix: Simplify localize tests - rely on fixture defaults and transactional isolation
* fix: Update system test to expect Spanish button text after locale preview
* refactor: Use I18n.t for button text in system test instead of hardcoded string
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Co-authored-by: Claude <noreply@anthropic.com>
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