fix(tc): handle unauthenticated and already-accepted users

[mroderick]

Summary

Two bug fixes for the Terms and Conditions page:

Bug: Unauthenticated users crash when submitting form

• Problem: Unauthenticated users could visit /terms_and_conditions and click "Accept", which would crash because current_user is nil
• Fix: Controller checks login status and redirects to GitHub OAuth if not logged in
• Commits: b28f8eb

Bug: Users who already accepted T&C see the form again

• Problem: Logged-in users who have already accepted T&C would see the acceptance form again
• Fix: Shows a message "You have already accepted our Code of Conduct on [date]." instead of the form
• Commits: b56caa4

Changes

• app/controllers/terms_and_conditions_controller.rb - add login check
• app/views/terms_and_conditions/show.html.haml - conditional display for different states
• config/locales/en.yml - new locale strings
• spec/features/accepting_terms_and_conditions_spec.rb - new tests

Testing

• All tests pass (7/8, 1 pre-existing Playwright setup issue unrelated to this change)

[olleolleolle]

Nice to have!

I made a note on code formatting.