feat(deadline-banner): auto mode pulls all calendar_items globally

Auto-mode deadline_banner now resolves to the next future event across
all calendar_items in the CMS instead of requiring a manually-curated
items[] pool. Server load hooks (+page.server.ts for /, /[...slug],
/post/[slug]) call resolveDeadlineBannerBlocks which fetches the
global calendar_item list once and attaches it as items[] when mode
is "auto" and items[] is not already resolved. Component logic
unchanged — it still picks the soonest future entry.

Also:
- cms.ts: getCalendarItems() bulk list fetcher
- PostOverviewBlock.svelte: design defaults to "cards" when value is
  empty string (not just null/undefined) so existing entries with
  design: "" render

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/block-types.ts

@@ -191,6 +191,30 @@ export interface PostOverviewBlockData {
   postsResolved?: unknown[];
 }
 
+/** Deadline-Banner (_type: "deadline_banner"). Schmaler Balken unter Hero. */
+export interface DeadlineBannerBlockData {
+  _type?: "deadline_banner";
+  _slug?: string;
+  mode?: "manual" | "auto";
+  label?: string;
+  text?: string;
+  date?: string;
+  items?: Array<
+    | string
+    | {
+        _slug?: string;
+        title?: string;
+        terminZeit?: string;
+        description?: string;
+        link?: string | { url?: string; linkName?: string; newTab?: boolean; _slug?: string };
+      }
+  >;
+  link?: string | { url?: string; linkName?: string; newTab?: boolean; _slug?: string };
+  variant?: "accent" | "urgent" | "info";
+  showCountdown?: boolean;
+  layout?: BlockLayout;
+}
+
 /** Tag-Referenz (API liefert oft { _slug, _type, name }). */
 export type SearchableTextTagRef = string | { _slug?: string; name?: string };