#21: guard the web surfaces and connect them to the auth service

Three separate gaps, all of which made the F4 interface non-functional or
unprotected:

The admin console rendered for anyone. There was no middleware and no
guard in the layout, so /admin/dashboard returned 200 unauthenticated -
only its data calls failed. middleware.ts now redirects both surfaces to
their login when the session cookie is missing, expired, carries the wrong
audience, or is a pre-MFA challenge token. It reads the claims without
verifying the signature on purpose: the signing key belongs to the auth
service and must not reach the browser bundle. That is sound because this
is not the boundary - every /api/v1 request is verified for real upstream.

Nothing served /api/v1/*. next.config.ts had no rewrites and there were no
route handlers, so every call the pages made 404'd against the Next
server, while AUTH_SERVICE_URL sat unused. A catch-all route now proxies
to the auth service, forwarding the client address and preserving multiple
Set-Cookie headers, which logout needs to clear both surfaces. Session
cookies also move to Path=/, since a cookie pinned to /user or /admin was
never sent with the /api/v1 requests that needed it.

i18n existed only as files. The three bundles had perfect key parity and
real translations, but no library, no imports and no switcher: the UI was
hardcoded English and i18n.test.ts compared keys while the feature was
absent. There is now a provider, a hook, a language switcher, and
server-side Accept-Language negotiation that honours q-values and matches
a bare "pt" to pt-BR. Logins, both layouts and the settings screen consume
it; the admin pages still have English titles.

The settings screen invented app passwords in the browser with Math.random
and never stored them. It now calls the API, which returns the password
once and keeps only its hash.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/webmail/src/middleware.ts

@@ -0,0 +1,80 @@
+import { NextResponse, type NextRequest } from "next/server";
+
+/**
+ * Gate for the two web surfaces.
+ *
+ * Without this, /admin/* rendered for anyone who typed the URL: the console
+ * was reachable unauthenticated and only its data calls failed. The check here
+ * is deliberately shallow - it reads the session cookie and its audience, and
+ * does not verify the signature, because the signing key belongs to the auth
+ * service and must not be copied into the browser-facing bundle.
+ *
+ * That is sound because this is not the security boundary: every /api/v1/*
+ * request is verified for real by the auth service, which owns the key. This
+ * only stops an unauthenticated visitor from loading a shell that cannot work,
+ * and sends them somewhere useful instead.
+ */
+
+const USER_COOKIE = "lm_user_session";
+const ADMIN_COOKIE = "lm_admin_session";
+
+interface JwtClaims {
+  aud?: string;
+  surface?: string;
+  purpose?: string;
+  exp?: number;
+}
+
+/** Reads the claims without verifying the signature. See the note above. */
+function readClaims(token: string | undefined): JwtClaims | null {
+  if (!token) return null;
+  const parts = token.split(".");
+  if (parts.length !== 3) return null;
+  try {
+    const payload = parts[1].replace(/-/g, "+").replace(/_/g, "/");
+    const json = atob(payload.padEnd(payload.length + ((4 - (payload.length % 4)) % 4), "="));
+    return JSON.parse(json) as JwtClaims;
+  } catch {
+    return null;
+  }
+}
+
+function isUsable(claims: JwtClaims | null, expectedAud: string): boolean {
+  if (!claims) return false;
+  // A challenge token is issued between the password and the second factor;
+  // it must not open either surface.
+  if (claims.purpose !== "session") return false;
+  if (claims.aud !== expectedAud) return false;
+  if (typeof claims.exp === "number" && claims.exp * 1000 <= Date.now()) return false;
+  return true;
+}
+
+export function middleware(req: NextRequest) {
+  const { pathname, search } = req.nextUrl;
+
+  const isAdminArea = pathname.startsWith("/admin") && !pathname.startsWith("/admin/login");
+  const isUserArea = pathname.startsWith("/user") && !pathname.startsWith("/user/login");
+  if (!isAdminArea && !isUserArea) {
+    return NextResponse.next();
+  }
+
+  const surface = isAdminArea ? "admin" : "user";
+  const cookieName = isAdminArea ? ADMIN_COOKIE : USER_COOKIE;
+  const claims = readClaims(req.cookies.get(cookieName)?.value);
+
+  if (isUsable(claims, `lambdamail:${surface}`)) {
+    return NextResponse.next();
+  }
+
+  // The original destination travels along so the login can return there,
+  // rather than always dropping the user on a default page.
+  const login = req.nextUrl.clone();
+  login.pathname = `/${surface}/login`;
+  login.search = "";
+  login.searchParams.set("next", `${pathname}${search}`);
+  return NextResponse.redirect(login);
+}
+
+export const config = {
+  matcher: ["/admin/:path*", "/user/:path*"],
+};