fix(web): wire up next-intl i18n — install dep, add locale middleware, wrap next config

The i18n architecture (config, routing, translation files, locale pages) was
already built but non-functional due to three missing pieces:
1. next-intl not listed in package.json
2. middleware.ts not using createMiddleware from next-intl/middleware
3. next.config.js not wrapped with createNextIntlPlugin

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/next.config.js

@@ -1,4 +1,7 @@
 const { withSentryConfig } = require('@sentry/nextjs');
+const createNextIntlPlugin = require('next-intl/plugin');
+
+const withNextIntl = createNextIntlPlugin('./i18n/request.ts');
 
 /** @type {import('next').NextConfig} */
 const nextConfig = {
@@ -44,7 +47,7 @@ const nextConfig = {
   },
 };
 
-module.exports = withSentryConfig(nextConfig, {
+module.exports = withSentryConfig(withNextIntl(nextConfig), {
   org: process.env.SENTRY_ORG,
   project: process.env.SENTRY_PROJECT,
   silent: !process.env.CI,