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/cms.ts

@@ -666,3 +666,20 @@ export async function getCalendarItemBySlug(
     return (await res.json()) as CalendarItemEntry;
   });
 }
+
+/** Alle calendar_item-Einträge (GET /api/content/calendar_item). */
+export async function getCalendarItems(
+  options?: { locale?: string },
+): Promise<CalendarItemEntry[]> {
+  const key = `calendar_item:list:${options?.locale ?? ""}`;
+  return cached("calendar_item", key, async () => {
+    const base = getBaseUrl();
+    const url = new URL(`${base}/api/content/calendar_item`);
+    url.searchParams.set("_per_page", "1000");
+    if (options?.locale) url.searchParams.set("_locale", options.locale);
+    const res = await fetch(url.toString());
+    if (!res.ok) return [];
+    const data = (await res.json()) as { items?: CalendarItemEntry[] };
+    return data.items ?? [];
+  });
+}