Issue #3595227: French demo polish -- landing-page translation + language-switcher styling

Consolidated French-demo polish (base preview only; gate currently TEMP-open to mergerequest for preview, reverts to === branch before merge):

1. Translate the landing page to French via content_translation (reloads the node after enabling translation so addTranslation sees the content_translation_* fields; guarded so MR previews without the language module do not fatal).
2. Language switcher polish: native names (English / Francais) and inline/no-bullets/active-bold styling via a tiny demo-only module (pdv_demo_lang) shipped under .tugboat/ and copied into modules/custom by the build -- no CSS added to the pdv module.

Verified end-to-end on a clean DB: landing node carries an fr translation, switcher module enables + library resolves, fr label = Francais; phpcs clean.