Feat/search endangered filter

[zigzagdev]

PR Content

• Adds the Endangered (危機遺産) filter to the heritage search form: a single checkbox that toggles is_endangered=true on the URL and wires it through the domain types, URL parse/serialize, search
  API, and form/result containers.
• Fixes a long-standing bug where ?lang=ja was dropped on every search-related navigation, so the search results page reverted to English. Form submit and all four navigations on the results page
  (pagination, re-submit, item click, back-to-all) now carry lang=ja forward when the user is in Japanese mode.
• Adds a LocaleToggle to the search results page header and pulls "Back to all sites" / "No sites found." from the ui-text dictionary so the page actually reflects the locale switch. The h1 brand
  "Search Results" and its subtitle stay untranslated by design (matches the World Heritage brand pattern in TopPageTitleBar).

What changed

Endangered filter

• HeritageSearchParams gains is_endangered: boolean | null; SearchValues gains isEndangered: boolean.
• parseHeritageSearchParams reads is_endangered=true (anything else → null); serializeHeritageSearchParams emits the param only when true. Round-trip covered by new
  search-heritages.params.test.ts.
• SearchParams (API client) and useHeritageSearchQuery thread isEndangered to the backend as is_endangered=true.
• HeritageSearchForm renders a checkbox above the year/keyword row; SearchHeritageFormContainer, SearchHeritageResultsContainer, and HeritageDetailLayout round-trip the field through their drafts
  and submit handlers.
• endangeredOnly ui-text key added to en (Endangered only) and ja (危機遺産のみ表示).

Lang preservation on search navigations

• SearchHeritageFormContainer.handleSubmit reads the current URL's lang and re-appends it to the next /heritages/results URL.
• SearchHeritageResultsContainer gets a small file-local preserveLang(nextSearch, currentSearch) helper used by handleClickItem / handlePageChange / handleSubmit / handleBackToAllSites.
  lang=en (default) is never written; only lang=ja is propagated.

SearchResultsPage localization

• <LocaleToggle /> placed next to "Back to all sites" so users can switch locale from the results page itself.
• Button label and empty-state ("No sites found.") read from useText(). New keys: backToAllSites, noSitesFound.

Tests

• npm run typecheck passes
• npm test passes (14 suites / 74 tests, including new search-heritages.params.test.ts)
• Manual: visit /heritages?lang=ja, open the search form, tick the 危機遺産のみ表示 checkbox, submit. Confirm the result URL becomes /heritages/results?is_endangered=true&lang=ja and the page is in Japanese.
• Manual: on the results page in ja, click pagination / re-submit the form / click an item / click 一覧に戻る. lang=ja survives every transition.
• Manual: visit /heritages/results?is_endangered=true directly (no lang). Click the 🇬🇧 toggle in the page header — UI flips to ja, URL gains &lang=ja, and reload preserves it.

[zigzagdev]

Ok