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/components/ui/Button.tsx

@@ -7,10 +7,24 @@ interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
   variant?: 'primary' | 'secondary' | 'outline' | 'ghost' | 'danger';
   size?: 'sm' | 'md' | 'lg';
   isLoading?: boolean;
+  /** Label shown in place of the children while loading. */
+  loadingText?: string;
 }
 
 const Button = forwardRef<HTMLButtonElement, ButtonProps>(
-  ({ className, variant = 'primary', size = 'md', isLoading, children, disabled, ...props }, ref) => {
+  (
+    {
+      className,
+      variant = 'primary',
+      size = 'md',
+      isLoading,
+      loadingText = 'Loading...',
+      children,
+      disabled,
+      ...props
+    },
+    ref
+  ) => {
     return (
       <button
         ref={ref}
@@ -62,7 +76,7 @@ const Button = forwardRef<HTMLButtonElement, ButtonProps>(
                 d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"
               />
             </svg>
-            Loading...
+            {loadingText}
           </>
         ) : (
           children