fix: final-review findings — own-lead descriptions, PHS integration landing, planned tiles, 404, sitemap math, cleanups

src/pages/404.astro

@@ -0,0 +1,20 @@
+---
+import Base from '../layouts/Base.astro';
+---
+<Base title="CTAO Science Portal — Page not found" description="This page does not exist">
+  {/* Document archetype (DESIGN.md): the document IS the page — h1, one line,
+      recovery links, and a real GET search form as the way forward. */}
+  <article class="container article">
+    <h1>Page not found</h1>
+    <div class="prose">
+      <p>The address may be mistyped or the page may have moved — go to the
+        <a href="/">home page</a> or browse <a href="/news">all news</a>.</p>
+    </div>
+    <form class="search-form" role="search" action="/search" method="get">
+      <label class="sr-only" for="nf-q">Search news</label>
+      <svg class="ico search-ico" aria-hidden="true" viewBox="0 0 24 24"><circle cx="11" cy="11" r="7" /><path d="m20 20-4.3-4.3" /></svg>
+      <input id="nf-q" name="q" type="search" placeholder="Search news…" autocomplete="off" />
+      <button class="btn" type="submit">Search</button>
+    </form>
+  </article>
+</Base>