chore(i18n): drop 191 dead translation keys from all locales (#3867)

Static-analysis sweep against literal t() calls, dynamic key construction,
pluralization variants, and substring use in lookup tables / property-access
scripts. 178 keys had no reference in src/, scripts/, server/, shared/, api/,
public/, or tests/, plus 13 stale extras that existed only in non-English
translations (8 components.economic.* + panels.{liveYouTube,pinnedWebcams}
universally, 5 more under deckgl.layers / modals.countryIntel in nl/pt/sv).
translate-locales.mjs would otherwise re-add them on next backfill.

Coverage: all 22 locales (en + 20 originals + hu added in #3857).

Preserved: shell.offline{Title,Message,Retry} (read by
scripts/sync-offline-translations.mjs into public/offline.html) and the
components.cii._methodologyLink_translatorNote sidecar.

Test-contract: drop 'createWithAi' and 'changeAccent' from REQUIRED_KEYS in
tests/widget-builder.test.mjs — both keys lost their last production
reference in 80c3751be and 47f0dd133 respectively; the contract was stale.

Verification: all 22 JSON files validate; widget-builder test 203/203;
sync-offline runs clean (no public/offline.html diff); all 21 locales
have exact en-parity except CLDR plural-cohort siblings (signals_few/_many/
_zero etc) added by #3908, which are legitimate.

Co-authored-by: Elie Habib <elie@worldmonitor.app>