feat(web-i18n): add es/fr locales and expand switcher locale set

docs/product-engineering/i18n-baseline.md

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+# i18n Baseline
+
+## Scope
+
+MVP0 introduces i18n runtime only for the public app (`@cms/web`) using `next-intl`.
+
+Current baseline:
+
+- Shared locale contract in `@cms/i18n` (`de`, `en`, `es`, `fr`; default `en`)
+- Path-stable routing (no locale in URL) via `apps/web/src/proxy.ts`
+- Message loading through `apps/web/src/i18n/request.ts`
+- Locale-aware navigation helpers in `apps/web/src/i18n/navigation.ts`
+- Public language switcher component backed by Zustand store
+
+## Notes
+
+- Public app locale is resolved through `next-intl` middleware + cookie.
+- Enabled locales are currently static in code and will later be managed from admin settings.
+- Admin app i18n provider/message loading is still pending.
+- Translation key conventions and workflow docs are tracked in `TODO.md`.