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)/auth/magic-link/page.tsx

@@ -7,6 +7,7 @@ import { useAuth } from '@/context/AuthContext';
 import Card from '@/components/ui/Card';
 import Button from '@/components/ui/Button';
 import { safeInternalPath } from '@/lib/safeRedirect';
+import { redirectAfterAuth } from '@/lib/authRedirect';
 import toast from 'react-hot-toast';
 
 function MagicLinkContent() {
@@ -46,7 +47,7 @@ function MagicLinkContent() {
           ? '/auth/claim-account'
           : callbackURL;
       setTimeout(() => {
-        router.push(destination);
+        redirectAfterAuth(destination);
       }, 1500);
     } catch (err: any) {
       setStatus('error');