feat(web): Next.js 15 app shell, auth, i18n, base UI

App Router setup with next-intl (en/fr), Better Auth wiring, shadcn/ui components,
Tailwind, AES-256-GCM encrypt util, Redis rate limiter, tier limit checker,
site_settings helper for runtime .env overrides.

apps/web/lib/auth/client.ts

@@ -0,0 +1,13 @@
+"use client";
+
+import { createAuthClient } from "better-auth/react";
+
+export const authClient = createAuthClient();
+
+export const {
+  signIn,
+  signUp,
+  signOut,
+  useSession,
+  getSession,
+} = authClient;