feat(i18n): translate the demo and add a demo language switcher

[ParadoxInfinite]

Brings the GitHub Pages demo into the translation layer and lets visitors switch
language from the demo itself.

What's here

• The demo components (banner, compare button, and the full comparison table) now
  resolve their text through the translation layer, so the demo is no longer
  English-only.
• A language switcher in the demo banner, shown once more than one language is
  available (so it appears automatically as catalogs get published).
• The demo has no backend to proxy catalogs, so in the demo build the locale
  loader reads the manifest and catalogs straight from the CDN (the same files the
  backend proxy serves in the real app). The real build is unchanged.

Only English is published today, so the switcher stays hidden until a translation
lands; the wiring is in place for when one does.

Testing

• npm run check-i18n passes (646 keys); a check confirms every t() key in the
  source resolves.
• Both the normal build and the VITE_DEMO=1 demo build compile; the committed
  web/dist is the normal (non-demo) build.