feat(layout): add global announcement banner above nav

Adds AnnouncementBanner component fed by a new top_banner CMS entry
(site-announcement). Extended top_banner schema and TS types with
optional link + linkLabel fields. Rendered inline-markdown text.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/cms.ts

@@ -747,6 +747,31 @@ export async function getTextFragmentBySlug(
 }
 
 /** Fullwidth-Banner (z. B. für Top-Banner mit Bild). */
+export type TopBannerEntry = components["schemas"]["top_banner"];
+
+/** Top-Banner anhand Slug (GET /api/content/top_banner/:slug). */
+export async function getTopBannerBySlug(
+  slug: string,
+  options?: { locale?: string },
+): Promise<TopBannerEntry | null> {
+  const key = `top_banner:slug:${slug}:${options?.locale ?? ""}`;
+  return cached("top_banner", key, async () => {
+    const base = getBaseUrl();
+    const url = new URL(
+      `${base}/api/content/top_banner/${encodeURIComponent(slug)}`,
+    );
+    if (options?.locale) url.searchParams.set("_locale", options.locale);
+    const res = await cmsFetch(url.toString());
+    if (res.status === 404) return null;
+    if (!res.ok) {
+      throw new Error(
+        `RustyCMS get top_banner failed: ${res.status} for slug "${slug}".`,
+      );
+    }
+    return (await res.json()) as TopBannerEntry;
+  });
+}
+
 export type FullwidthBannerEntry = components["schemas"]["fullwidth_banner"];
 
 /** Fullwidth-Banner anhand Slug (GET /api/content/fullwidth_banner/:slug). */