Fix post-login redirect when the session cookie is off-origin

Signing in showed the "Welcome back!" toast but never left /login. The
session cookie was host-only on the API subdomain, so the Next middleware
guard on the site origin saw no cookie and bounced /dashboard straight
back to /login?redirect=/dashboard.

- Add AUTH_COOKIE_DOMAIN, wiring Better Auth's crossSubDomainCookies so
  the cookie also reaches the site origin. Unset in dev, where localhost
  is single-host and must stay host-only.
- Navigate after authentication with a full page load, via a shared
  authRedirect helper: only a top-level request carries the httpOnly
  cookie. Used by the login, register, magic-link and Google flows.
- Show "Redirecting..." on the login and register pages and keep the
  submit button disabled until the browser replaces the page, instead of
  re-enabling it mid-navigation.
- Guard against a redirect loop with a sessionStorage marker. A React ref
  cannot do this: the full page load resets component state. If the
  destination bounces back, explain it rather than navigating again.
- Middleware: accept any *.session_token cookie so a cookiePrefix change
  cannot lock everyone out, and preserve the destination's query string.
- Trust any loopback port in dev, so reaching the dev server through a
  forwarded port does not fail Better Auth's CSRF origin check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/app/(public)/register/page.tsx

@@ -1,7 +1,6 @@
 'use client';
 
 import { useState } from 'react';
-import { useRouter } from 'next/navigation';
 import Link from 'next/link';
 import { useLanguage } from '@/context/LanguageContext';
 import { useAuth } from '@/context/AuthContext';
@@ -9,13 +8,16 @@ import Card from '@/components/ui/Card';
 import Button from '@/components/ui/Button';
 import Input from '@/components/ui/Input';
 import GoogleSignInButton from '@/components/GoogleSignInButton';
+import { redirectAfterAuth } from '@/lib/authRedirect';
 import toast from 'react-hot-toast';
 
+const REDIRECT_TO = '/dashboard';
+
 export default function RegisterPage() {
-  const router = useRouter();
   const { t, locale: language } = useLanguage();
   const { register } = useAuth();
   const [loading, setLoading] = useState(false);
+  const [redirecting, setRedirecting] = useState(false);
   const [formData, setFormData] = useState({
     name: '',
     email: '',
@@ -30,10 +32,12 @@ export default function RegisterPage() {
     try {
       await register(formData);
       toast.success(language === 'es' ? 'Cuenta creada exitosamente!' : 'Account created successfully!');
-      router.push('/dashboard');
+      // Deliberately leaves `loading` set: the button must stay disabled until the
+      // browser replaces this page.
+      setRedirecting(true);
+      redirectAfterAuth(REDIRECT_TO);
     } catch (error: any) {
       toast.error(error.message || t('auth.errors.emailExists'));
-    } finally {
       setLoading(false);
     }
   };
@@ -104,9 +108,25 @@ export default function RegisterPage() {
                 onChange={(e) => setFormData({ ...formData, phone: e.target.value })}
               />
               
-              <Button type="submit" className="w-full" size="lg" isLoading={loading}>
+              <Button
+                type="submit"
+                className="w-full"
+                size="lg"
+                isLoading={loading || redirecting}
+                loadingText={redirecting ? t('auth.login.redirecting') : t('common.loading')}
+              >
                 {t('auth.register.submit')}
               </Button>
+
+              {redirecting && (
+                <p
+                  className="text-center text-sm text-gray-600"
+                  role="status"
+                  aria-live="polite"
+                >
+                  {t('auth.login.redirecting')}
+                </p>
+              )}
             </form>
 
             <p className="mt-6 text-center text-sm text-gray-600">