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-api.generated.ts

@@ -5446,6 +5446,10 @@ export interface components {
             id: string;
             /** @description Banner text (e.g. notice at top of page) */
             text: string;
+            /** @description Optional URL the banner links to */
+            link?: string;
+            /** @description CTA label for the link */
+            linkLabel?: string;
         };
         top_banner_input: {
             /** @description URL slug (used as filename) */
@@ -5454,6 +5458,10 @@ export interface components {
             id: string;
             /** @description Banner text (e.g. notice at top of page) */
             text: string;
+            /** @description Optional URL the banner links to */
+            link?: string;
+            /** @description CTA label for the link */
+            linkLabel?: string;
         };
         translation_bundle: {
             /** @description Entry identifier (filename) */