Revert "feat: login rewritten — split-screen auth (white form column + living aurora pane, tagline), 2026 convention"

This reverts commit 185380ffa5.

src/pages/login.astro

@@ -2,24 +2,16 @@
 import Base from '../layouts/Base.astro';
 ---
 <Base title="CTAO Science Portal — Sign in" description="Sign in via CTAO AAI (mock)">
-  {/* Split-screen auth (2026 convention: Linear/Stripe/Vercel) — clean white
-      form column + living brand aurora pane. Auth pages are scanned, not read:
-      one line of copy, one action. */}
-  <section class="auth">
-    <div class="auth-pane">
+  <section class="auth-band">
+    <div class="band-bg" aria-hidden="true"></div>
+    <div class="auth-card">
       <img class="auth-logo" src="/brand/CTAO_Logo_positive.svg" alt="CTAO" />
+      <span class="badge-mock">Mock — AAI integration pending</span>
       <h1>Sign in</h1>
+      {/* Auth pages are scanned, not read — one line, no reassurance prose */}
       <p class="muted">Use your CTAO account — single sign-on via CTAO AAI.</p>
       <button class="btn btn-wide" type="button">Continue with your CTAO account</button>
       <p class="notice">No account? <a>Register via CTAO AAI</a></p>
-      <p class="auth-foot">
-        <span class="badge-mock">Mock — AAI integration pending</span>
-        <a class="back" href="/">← Back to the portal</a>
-      </p>
-    </div>
-    <div class="auth-visual auth-band" aria-hidden="true">
-      <div class="band-bg"></div>
-      <p class="auth-tagline">Exploring the Universe<br />at the Highest Energies</p>
     </div>
   </section>
 </Base>