fix(auth): resolve MFA post-activation login failures — tickets #38 #40 #41

#41 (critical): Replace plain Error throws in authorize() with CredentialsSignin
subclasses (MfaRequiredError / MfaRequiredSetupError / InvalidTotpError).
Auth.js v5 forwards CredentialsSignin.code to the client via SignInResponse.code;
plain throws become CallbackRouteError and the message is never visible.
Signin page now checks result.code ?? result.error for exact code matching.

#38: MfaPromptBanner converted to fully client-side component via
trpc.user.getMfaStatus.useQuery() — disappears immediately after MFA enable
without requiring page reload. Snooze key remains userId-scoped via useSession().
Server-side prisma.user.findUnique call removed from (app)/layout.tsx.

#40: NEXTAUTH_URL default fallback removed from docker-compose.yml.
The variable is now required (:?) — docker compose up fails with a descriptive
error if the value is missing, preventing silent localhost redirect bugs.

Tests: auth.test.ts (5), MfaPromptBanner.test.ts (7), reset-password.test.ts (6)
All new tests green. pnpm --filter @capakraken/web exec tsc --noEmit clean.

Co-Authored-By: claude-flow <ruv@ruv.net>

apps/web/src/app/(app)/layout.tsx

@@ -1,5 +1,4 @@
 import { redirect } from "next/navigation";
-import { prisma } from "@capakraken/db";
 import { AppShell } from "~/components/layout/AppShell.js";
 import { MfaPromptBanner } from "~/components/security/MfaPromptBanner.js";
 import { auth } from "~/server/auth.js";
@@ -15,21 +14,10 @@ export default async function AppLayout({ children }: { children: React.ReactNod
 
   const sessionUser = session.user as { id?: string; email?: string; role?: string } | undefined;
   const userRole = sessionUser?.role ?? "USER";
-  const userId = sessionUser?.id;
-
-  // Show MFA prompt banner for privileged roles that haven't enabled TOTP yet
-  let showMfaPrompt = false;
-  if (userId && MFA_PROMPT_ROLES.has(userRole)) {
-    const user = await prisma.user.findUnique({
-      where: { id: userId },
-      select: { totpEnabled: true },
-    });
-    showMfaPrompt = user != null && !user.totpEnabled;
-  }
 
   return (
     <AppShell userRole={userRole}>
-      {showMfaPrompt && userId && <MfaPromptBanner userId={userId} />}
+      {MFA_PROMPT_ROLES.has(userRole) && <MfaPromptBanner />}
       {children}
     </AppShell>
   );