Bilingual content, end to end

The site was already bilingual in its routes. Now the content is too, and a check stops a half-translated page from shipping.

Added

• src/content/{en,fr}/ collections for blog, legal and services, keyed by a shared slug so the two locales stay in step.
• A language switcher that swaps the locale segment and keeps you on the same page instead of dropping you at the home page.
• check:content, which fails the build when a slug exists in one locale and not the other.

Changed

• Dates render through a single FormattedDate component, so en and fr get their own month names without a per-page format string.

Fixed

• The sitemap listed only the default-locale routes.