AO3-7399 Add username to Challenge Assignments page title

[sagittaerys]

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• Have you read the contributing guidelines?
• Have you added tests for any changed functionality?
• Have you added the Jira issue number
  as the first thing in your pull request title (e.g. AO3-1234 Fix thing)
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Issue

https://otwarchive.atlassian.net/browse/AO3-7399

Purpose

Adds the username to the Challenge Assignments page title to be
consistent with other user pages such as Works and Challenge Claims.

Root cause:
The index action in ChallengeAssignmentsController was not
setting @page_subtitle for the user assignments view, unlike
similar pages across the app.

Changes made:

• Added @page_subtitle to the user assignments branch of the index action in app/controllers/challenge_assignments_controller.rb
• Added the corresponding i18n key to config/locales/controllers/en.yml

Testing Instructions

1. Log in as any user
2. Navigate to /users/{username}/assignments
3. Verify the browser tab shows "username - Challenge Assignments | Archive of Our Own"
4. Verify visiting another user's assignments page redirects with an error

Automated tests:

• Added a new RSpec test in spec/controllers/challenge_assignments_controller_spec.rb

Command to run:
docker compose run --rm test bundle exec rspec spec/controllers/challenge_assignments_controller_spec.rb

Credit

Name: sagittaerys (Olamilekan Aremu)
Pronouns: (he/him)

[Bilka2]

Hi, Olamilekan Aremu!

Thank you so much for this pull request. Someone will be along to review it soon.

In the meantime, someone updated the Jira issue status to In Review so no one mistakenly creates a duplicate pull request. If you'd like the ability to comment on, assign, and transition issues in the future, you're welcome to create a Jira account! It makes things a bit easier for us on the organizational side if the Full Name on your Jira account either closely matches the name you'd like us to credit in the release notes or includes it in parentheses, e.g. "Nickname (CREDIT NAME)."

Once you've done that (or if you've already done it -- Jira has been unreliable about showing us new accounts in the admin panel lately), you can either reply here or send an email to otw-coders@transformativeworks.org with your account name and email address and we'll set up the permissions for you.

Thanks again for contributing! If you have any questions, you can contact us at the same email address listed above.

[Bilka2]

Thank you!

[sagittaerys]

Can I take on other tasks?

[sarken]

@sagittaerys Yup, you're absolutely welcome to take on additional issues!