feat: Translate config entities (menus) via config overrides

Lauri Timmaneerequested to merge
issue/jsonapi_multilingual-3612368:3612368-translate-menus-across into 1.0.x

DIVERGENCE(core): #3199697 translates content entities only. This applies config language overrides (e.g. a menu label) through the same EntityAccessChecker::getEntityTranslation() seam, so a config resource is returned in the requested language. langCode is strict: a config entity with no override for that language 404s (individual) or is excluded (collection, filtered in loadEntitiesWithAccess since config has no query-filterable translation rows); Accept-Language falls back to the default.

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

Closes #3612368

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