feat: portal IA per spec (home entry point, /news, /proposals+redirect, /support), live search + suggestions, article TOC, mobile disclosure menu, CMS preview styles, modernization pass 1

src/pages/login.astro

@@ -0,0 +1,17 @@
+---
+import Base from '../layouts/Base.astro';
+---
+<Base title="CTAO Science Portal — Sign in" description="Sign in via CTAO AAI (mock)">
+  <section class="auth-band">
+    <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>
+      <p class="muted">One account for all CTAO services. Authentication is handled by the
+      central <strong>CTAO AAI</strong> (single sign-on) — the portal never sees your password.</p>
+      <button class="btn btn-wide" type="button">Continue with your CTAO account</button>
+      <p class="notice">No account yet? <a>Register via CTAO AAI</a> · Access level follows
+      your CTAO role and permissions.</p>
+    </div>
+  </section>
+</Base>