feat: add carousel design to PostOverview; extract shared Carousel component

- New Carousel.svelte: index-fade, arrows, dots, optional auto-rotate
- QuoteCarouselBlock refactored to use Carousel
- PostOverviewBlock: carousel design variant added
- CMS schema: "carousel" added to post_overview design enum

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

src/lib/block-types.ts

@@ -236,8 +236,8 @@ export interface PostOverviewBlockData {
   excludeTag?: string[];
   /** Max. Anzahl Einträge. */
   numberItems?: number;
-  /** "list" | "cards". */
-  design?: "list" | "cards";
+  /** "list" | "cards" | "carousel". */
+  design?: "list" | "cards" | "carousel";
   layout?: BlockLayout;
   /** Nach resolvePostOverviewBlocks: geladene/gefilterte Posts (PostEntry[]). */
   postsResolved?: unknown[];