i18n — web marketing-route i18n (/fr URL routing + FR legal MDX + SEO hreflang)

Split from Phase 3b (200b0537). Web marketing-route i18n (items 1–3), to run AFTER the string sweep is well underway. Builds on the merged cookie-driven core.

1. URL-prefixed [locale] marketing routes — / = en, /fr/...; restructure app/[locale]/(marketing)/...; keep /app/* cookie-driven (no prefix). Compose with the existing locale-aware apps/web/middleware.ts (order: detect→auth→redirect); verify auth redirects for /fr/login. Touches middleware → review-gated, no auto-merge.
2. FR legal MDX — parallel fr/ MDX legal pages under app/[locale]/(legal)/.
3. SEO — sitemap.ts/robots.ts hreflang + per-locale marketing URLs + per-locale OG/metadata.

Also: once /fr/... URLs exist, add a prominent nav language switcher (the picker currently lives in the footer + /app settings).