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/components/AnnouncementBanner.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+  import type { TopBannerEntry } from '$lib/cms';
+  import { marked } from '$lib/markdown-safe';
+
+  let { banner }: { banner: TopBannerEntry } = $props();
+
+  const textHtml = $derived(banner?.text ? marked.parseInline(banner.text) as string : '');
+</script>
+
+{#if banner?.text}
+  <div class="w-full bg-wald-600 text-white print:hidden" role="note" style="font-size: 0.7rem;">
+    <div class="container-custom flex items-center gap-3 py-2 md:justify-center md:text-center">
+      <span>{@html textHtml}</span>
+      {#if banner.link}
+        <a
+          href={banner.link}
+          class="shrink-0 font-semibold underline underline-offset-2 hover:no-underline"
+        >
+          {banner.linkLabel ?? 'Mehr erfahren'} →
+        </a>
+      {/if}
+    </div>
+  </div>
+{/if}