fix(auth): only the back-to-home link returns to the landing page

The brand logo on the auth pages now points to the login page instead
of the landing; reaching the public landing is done solely via the
explicit "back to home" link.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/auth/AuthShell.tsx

@@ -45,7 +45,9 @@ export function AuthShell({
 
       {/* Header */}
       <header className="relative z-10 mx-auto flex max-w-5xl items-center justify-between px-6 py-5">
-        <Link href="/" aria-label={t.common.appName} className="transition hover:opacity-90">
+        {/* Brand returns to the auth entry; only the explicit "back to home"
+            link leaves for the public landing page. */}
+        <Link href="/login" aria-label={t.common.appName} className="transition hover:opacity-90">
           <Logo className="abrban-ink" />
         </Link>
         <div className="flex items-center gap-1">