fix(setup): Apply language choice and fix post-setup redirect

- Call changeLanguage(values.language) after successful owner creation so
  the UI immediately switches to the language the owner chose
- Call _markSystemInitialized() to update the router cache before navigating
  to /login; without this the guard saw initialized=false and redirected
  back to /setup, blocking the redirect

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

frontend/src/pages/SetupPage.tsx

@@ -12,6 +12,8 @@ import { FormErrorBanner } from '@/components/ui/FormErrorBanner';
 import { FieldError } from '@/components/ui/FieldError';
 import { useSetup } from '@/api/useSetup';
 import { ProblemDetailsError, NetworkError } from '@/lib/api-client';
+import { changeLanguage } from '@/i18n/config';
+import { _markSystemInitialized } from '@/router';
 
 interface FormError {
     message: string;
@@ -47,6 +49,8 @@ export function SetupPage() {
         try {
             const { confirmPassword: _, ...payload } = values;
             await setupMutation.mutateAsync(payload);
+            await changeLanguage(values.language);
+            _markSystemInitialized();
             setSuccessMessage(true);
             setTimeout(() => {
                 navigate({ to: '/login' });

frontend/src/router.tsx

@@ -50,6 +50,11 @@ export function _resetSetupStatusCache(): void {
     _setupStatusPromise = null;
 }
 
+export function _markSystemInitialized(): void {
+    _setupStatusCache = { initialized: true };
+    _setupStatusPromise = null;
+}
+
 function RouteFallback() {
     return (
         <div className="flex min-h-40 items-center justify-center text-muted-foreground">