Translate the admin interface (i18n for the back-office) #218

Today only the public booking pages are translatable (internal/i18n/locales). The admin back-office (SvelteKit SPA in frontend/) is English-only, so non-English operators can't use Calnode in their language.

#217 is a first step: it adds an admin i18n catalog (frontend/src/lib/i18n, en.json as source of truth), a locale picker in Settings → Profile, locale-aware dates and plurals (Intl.PluralRules), and a complete French translation.

With this in place, other languages only need a new catalog file. For example, the Polish strings proposed in #216 cover the public pages, and a pl.json for the admin could follow the same structure.

Related: #216, #217