feat: render current spec on index page instead of redirecting

- Update index.astro to fetch and render the current spec version
  directly rather than redirecting to /spec/{version}
- Move .spec-content styles from [version].astro to global.css
  to share styles between index and version pages
- Rebuild site with new index page

src/pages/index.astro

@@ -1,6 +1,21 @@
 ---
+import { getCollection, render } from "astro:content";
+import Default from "../layouts/Default.astro";
 import { config } from "../config";
 
-// Redirect to current version
-return Astro.redirect(`/spec/${config.currentVersion}`);
+// Get all specs and find the current version
+const specs = await getCollection("spec");
+const currentSpec = specs.find((s) => s.data.version === config.currentVersion);
+
+if (!currentSpec) {
+  return Astro.redirect("/404");
+}
+
+const { Content } = await render(currentSpec);
 ---
+
+<Default title={currentSpec.data.title} version={currentSpec.data.version}>
+  <article class="spec-content">
+    <Content />
+  </article>
+</Default>