Upgrade React Router to v8

[sneridagh]

Summary

Upgrades React Router from 7.14.0 to 8.2.0 (react-router, @react-router/dev, @react-router/node, @react-router/serve), and remix-i18next from 7.1.0 to 8.0.0 (required, since remix-i18next only supports React Router v8 starting at its own v8).

React Router v8's breaking changes are mostly the codification of flags that were already opt-in in v7 (future.v8_middleware, which this app had already adopted), plus a couple of renamed fields. The bulk of this PR is migrating off remix-i18next v7's RemixI18Next class, which was removed in v8 in favor of a middleware-based API.

React Router v8 changes

• unstable_pattern / unstable_url → pattern / url: renamed (now stable, unprefixed) across loader/action test helpers in apps/aurora, packages/cmsui, and packages/contents.
• MetaMatch.data → MetaMatch.loaderData: updated meta() functions in cmsui, contents, and publicui root layouts/routes that read the root route's loader data from matches.
• Removed the now-default future.v8_middleware flag from apps/aurora/react-router.config.ts.
• Bumped apps/aurora engines to Node >=22.22.0 (v8's minimum).

remix-i18next v8 migration

• Rewrote apps/aurora/app/i18next.server.ts to use createI18nextMiddleware, replacing the removed RemixI18Next class and its /server, /client, /react subpath exports. It now exports [i18nextMiddleware, getLocale, getInstance].
• Registered i18nextMiddleware in root.tsx's middleware array so getLocale(context) / getInstance(context) work from route context, and updated the cmsui, contents, and publicui root layouts that independently called i18next.getLocale(request) to use the new context-based getLocale.
• Simplified entry.server.tsx to render with the middleware-provided instance (getInstance(loadContext)) instead of manually creating and initializing a per-request i18next instance.
• Replaced the removed useChangeLanguage hook in root.tsx's Layout with an equivalent useTranslation + useEffect sync.
• Important fix: defaultNS and ns are independent i18next options. The removed getInitialNamespaces()/getRouteNamespaces() helpers previously supplied an explicit ns array computed from matched routes. Simply dropping them (relying on defaultNS: 'common' alone) would silently make i18next preload the wrong translation namespace instead of common, since ns defaults independently to ['translation']. Fixed by explicitly setting ns: i18n.defaultNS in both the client (entry.client.tsx) and server (i18next.server.ts) init options.

Dependency note: prettier bump (3.6.2 → 3.9.5)

@react-router/dev@8.2.0 has a hard dependency on prettier: ^3.8.3, which the repo's previously-resolved prettier@3.6.2 (satisfying packages/tooling's ^3.5.2 range) can no longer share a single resolved version with. pnpm's natural resolution bumps the shared prettier install to 3.9.5 to satisfy both ranges. This is a required consequence of the upgrade, not incidental drift — earlier revisions of this PR mistakenly tried to pin it back down; that's been reverted. Note that prettier 3.9.5 has some formatting-rule differences from 3.6.2, so running prettier --write repo-wide after this merges may produce unrelated formatting diffs on files untouched by this PR; that's expected and best handled as its own follow-up formatting pass rather than bundled here.

Test plan

• pnpm install — remix-i18next peer warning against react-router is gone
• Typecheck clean across apps/aurora, packages/cmsui, packages/publicui, packages/layout, packages/react-router (packages/contents has pre-existing, unrelated type errors not touched by this change)
• Full test suites pass: apps/aurora (44/44), packages/cmsui (178/178), packages/contents (3/3), packages/layout (8/8), packages/react-router (5/5)
• Production build succeeds (pnpm build)
• Smoke-tested pnpm start:prod boot: middleware chain, routing, and SSR run end-to-end (the resulting 500 is expected — no local Plone backend was running in the test environment; it renders via the app's own ErrorBoundary, confirming the request pipeline completed)
• prettier/eslint clean on all changed files