Issue #3595227: Polish the demo language switcher (native names + styling) [preview-on-MR]

Frank Mablyrequested to merge
issue/pdv-3595227:3595227-language-switcher-polish into 1.x

Polishes the demo language switcher (follow-up to the French demo work):

  • Native language names: the French language entity label is set to "Français" (not the English "French"), so the switcher reads English / Français.
  • Inline, no-bullets layout with the active language in bold. The CSS is NOT added to the pdv module: a tiny demo-only module (pdv_demo_lang) ships under .tugboat/, is copied into modules/custom by the build (before site:install so it is discovered), and attaches a small CSS library site-wide via hook_page_attachments. Targets #block-pdv-demo-language-switcher a.is-active (core active-link JS marks the current language).

TEMP: the language gate is opened to per-MR previews so this can be eyeballed on the MR preview. Reverts to base-only (=== branch) before merge.

Verified locally: module enables, library resolves, fr label = Français; phpcs clean.

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