CTAO Science Portal: static Astro site with git-based CMS (Sveltia + Gitea)

Portal code only; editorial content lives in the ctao/content repo on the
demo machine's Gitea and is overlaid at build time (see README.md and
deploy/README.md). Live demo: https://astro.isl-dev.grid.cyfronet.pl

src/pages/404.astro

@@ -0,0 +1,24 @@
+---
+import Base from '../layouts/Base.astro';
+---
+<Base title="CTAO Science Portal - Page not found" description="This page does not exist">
+  {/* Document archetype: the document IS the page — h1, one line,
+      recovery links, and a real GET search form as the way forward. */}
+  <div class="container article-layout">
+    <article class="article">
+      <h1>Page not found</h1>
+      <div class="prose">
+        {/* Keep "the <a>" on one line: the compiler drops the whitespace at a
+            line break before a tag, which glued "the" to the link text. */}
+        <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>
+  </div>
+</Base>