Add temporary pre-catalog Translatathon promo banner

[iHiD]

What

A temporary announcement bar recruiting translators for the Big Jiki & Exercism Translatathon, shown to signed-in users whose browser's top language is one we don't ship the UI in.

This is the server-side sibling of the Jiki front-end's TranslatathonBanner. Same core logic, adapted to how this site does banners: it's a Ruby announcement bar in the site-header helper, not a React component.

How it works

• ViewComponents::TranslatathonBanner resolves the ordered Accept-Language request header (the server-side equivalent of the front-end's navigator.languages, ordered by q-value).
• It takes the first non-catalog language in that list. English is the only catalog language on Exercism, so English speakers see nothing and fall through to the existing bars.
• A language in the Translatathon program gets the whole banner in that language (COPY map, RTL-aware for ar/fa/ur).
• Any other named non-English language gets the English fallback with its name filled in (e.g. "Want to help us translate Jiki to Swedish?"). Ruby has no Intl.DisplayNames, so names come from a LANGUAGE_NAMES map; a code we can't name shows nothing.
• Self-removes after the event (hardcoded END_TIME = 2026-08-04).
• The bar links to https://i18n.jiki.io, opening in a new tab.

Mounting

Wired into SiteHeader#announcement_bar, following the existing announcement-bar pattern. It takes priority over the other bars for signed-in users when it applies. Signed-out users are never affected (and their header stays cached).

Notes vs the front-end PR

• No dismissal / no client-side rendering. The announcement bar is server-rendered and non-dismissible by convention here, so the localStorage dismiss logic and full-screen-exercise suppression from the FE version are dropped.
• Temporary by design: delete TranslatathonBanner, its test, and the header hook after the event.

Unit-tested in test/helpers/view_components/translatathon_banner_test.rb (Accept-Language ordering, catalog exclusion, program/variant mapping, English fallback, end-date cutoff).

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