Redirect unsupported locale prefixes to English

[zandonella]

Fixes

Fixes #575 by @sarayourfriend

Description

Currently, visiting a path with an unsupported, locale-shaped prefix (e.g. openverse.org/cn) matches no route and renders a 404 page.

This PR adds a global route middleware that detects these paths, strips the locale-shaped prefix, and redirects to the English equivalent. On the page the visitor lands on, a one-time banner explains that Openverse isn't translated into the requested language yet and links to the project page where they can contribute a translation.

Details:

• getUnsupportedLocaleRedirect util detects a locale-shaped first path segment (two letters, optional hyphenated region — e.g. cn, es-ar) that isn't in the configured locale set, and returns the prefix-stripped (English) path.
• unsupported-locale.global middleware performs the redirect for unmatched routes and controls banner visibility.
• The ui store persists an unsupportedLocaleRedirect flag so it survives the server-side redirect, then consumes it on the landing page to show the transient banner once. The banner is hidden on any subsequent navigation.
• New VUnsupportedLocaleBanner component, wired into VBanners, with new notification.unsupportedLocale.* strings.

Testing Instructions

1. Run the frontend locally.
2. Visit an unsupported, locale-shaped path such as http://localhost:8443/cn (or /cn/search?q=cat).
3. Verify you are redirected to the English equivalent (/ or /search?q=cat) rather than seeing a 404, and that the "language not supported" banner appears once.
4. Navigate elsewhere and confirm the banner disappears and does not reappear.
5. Confirm a supported locale (e.g. /es) and a genuinely unknown path (e.g. /foobar) are unaffected.
6. Run the unit tests: ov just frontend/run test:unit unsupported-locale.

Checklist

• My pull request has a descriptive title (not a vague title likeUpdate index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.
• I ran the DAG documentation generator (ov just catalog/generate-docs for catalog
  PRs) or the media properties generator (ov just catalog/generate-docs media-props
  for the catalog or ov just api/generate-docs for the API) where applicable.

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