[FE-40] Set up next-intl i18n — English translations, language switcher in user settings

[mftee]

Overview

The frontend is entirely in English with no i18n infrastructure. As SMALDA targets international users, the UI should be translatable. next-intl is the recommended i18n library for Next.js App Router. This issue establishes the infrastructure and adds the first translation file.

Background

Files to create/modify:

• frontend/messages/en.json — English translation strings (all UI labels, button text, error messages)
• frontend/i18n.ts — next-intl configuration
• frontend/middleware.ts — extend existing middleware (FE-04) with locale detection
• frontend/app/[locale]/layout.tsx — locale-aware root layout

Language switcher:

• Dropdown in the user settings header (/profile or the navbar)
• Available languages: English (en), French (fr), Spanish (es) — with placeholder fr.json and es.json showing only the key structure

Integration with backend:

• When user changes language in the UI, call PATCH /api/users/me to save preferredLanguage
• The backend then uses this preference for emails (BE-86)

Acceptance Criteria

• next-intl configured and working in the App Router
• en.json covers all UI strings on at least the login, dashboard, and document list pages
• Placeholder fr.json and es.json created (can be copies of English initially)
• Language switcher changes the displayed language without a full page reload
• Language preference saved to the backend on change