feat(deps): next-intl scaffold (English-only, future locale-add ready)

Minimal next-intl wire-up so future i18n additions are a config
change, not a code rewrite. No URL routing changes — there's no
`/<locale>/` prefix because there's no second locale today.

- `src/i18n/request.ts` — request-scoped locale + messages loader,
  hard-coded to 'en'
- `messages/en.json` — common namespace with a few sample keys
- `next.config.ts` — withNextIntlPlugin wraps the config
- `src/app/layout.tsx` — wraps body with NextIntlClientProvider so
  client components can `useTranslations('common')` now

When a real locale target appears (Polish for marina users, Italian
for broker portal, etc.):
1. Add `messages/<locale>.json`
2. Move route folders under `app/[locale]/` to enable URL routing
3. Add a `routing.ts` with the locale list + default

Verified: tsc clean, vitest 1315/1315, next build green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

messages/en.json

@@ -0,0 +1,9 @@
+{
+  "common": {
+    "save": "Save",
+    "cancel": "Cancel",
+    "delete": "Delete",
+    "edit": "Edit",
+    "back": "Back"
+  }
+}