AO3-5047 Prevent duplicate AJAX flash messages

[noahbravo]

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Issue

https://otwarchive.atlassian.net/browse/AO3-5047

Purpose

Fixes duplicated flash messages for AJAX create/destroy actions (e.g. Favorite Tag) when another flash message/banner is already present on the page.

Root cause:

• The ajax-create-destroy handler used a broad selector (.flash), causing messages to be written into multiple flash containers.

Changes made:

• Scoped AJAX flash output to the main flash container only (#main > .flash) in public/javascripts/application.js.
• Added a guard clause to skip flash rendering if no main flash container is found.
• Updated the first-login help banner in app/views/users/show.html.erb from class="flash notice" to class="notice" so it is not treated as a flash target.

This keeps the fix minimal and aligned with the issue note about the first-login banner being a notice rather than a flash.

Testing Instructions

Follow Example 1 from AO3-5047:
https://otwarchive.atlassian.net/browse/AO3-5047

Expected behavior:

• After logging in from a tag works index page and pressing Favorite Tag (or Unfavorite Tag), the success message appears only once.
• The AJAX message should not duplicate into multiple blue banners.

Automated tests:

• Added a new @javascript regression scenario in features/tags_and_wrangling/favorite_tags.feature covering:
  • logged-out user on tag works index
  • login from header form (existing "Successfully logged in" flash present)
  • Favorite Tag action
  • assertion that the success message appears exactly 1 time

Commands run:

• docker compose run --rm test bundle exec cucumber features/tags_and_wrangling/favorite_tags.feature:42
• docker compose run --rm test bundle exec cucumber features/tags_and_wrangling/favorite_tags.feature

Additional manual spot-check:

• Tag page deep pagination case from Jira notes (e.g. high page number) should also show the AJAX success message only once.

Credit

Name: Noah Bravo
Pronouns: they/them