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/blog-utils.ts

@@ -6,6 +6,7 @@ import {
   getTextFragmentBySlug,
   getCalendarBySlug,
   getCalendarItemBySlug,
+  getCalendarItems,
 } from "./cms";
 import type { RowContentLayout } from "./block-types";
 import type { CalendarItemData } from "./block-types";
@@ -528,6 +529,60 @@ export async function resolveCalendarBlocks(
   }
 }
 
+function isDeadlineBannerBlock(
+  item: unknown,
+): item is {
+  _type?: string;
+  mode?: string;
+  items?: unknown[];
+} {
+  return (
+    typeof item === "object" &&
+    item !== null &&
+    (item as { _type?: string })._type === "deadline_banner"
+  );
+}
+
+/**
+ * Löst deadline_banner-Blöcke im auto-Modus auf: Lädt alle calendar_items
+ * global und setzt sie als Pool in block.items, sofern dort noch keine
+ * aufgelösten Einträge liegen. Die Komponente wählt daraus den nächsten
+ * zukünftigen Termin.
+ */
+export async function resolveDeadlineBannerBlocks(
+  layout: RowContentLayout,
+): Promise<void> {
+  const rows = [
+    layout.row1Content ?? [],
+    layout.row2Content ?? [],
+    layout.row3Content ?? [],
+  ];
+  let pool: CalendarItemData[] | null = null;
+  const loadPool = async (): Promise<CalendarItemData[]> => {
+    if (pool === null) {
+      const items = await getCalendarItems({ locale: "de" });
+      pool = items as unknown as CalendarItemData[];
+    }
+    return pool;
+  };
+  for (const content of rows) {
+    if (!Array.isArray(content)) continue;
+    for (const item of content) {
+      if (!isDeadlineBannerBlock(item)) continue;
+      if (item.mode !== "auto") continue;
+      const existing = item.items ?? [];
+      const first = existing[0];
+      const alreadyResolved =
+        typeof first === "object" &&
+        first !== null &&
+        "terminZeit" in first &&
+        "title" in first;
+      if (alreadyResolved) continue;
+      (item as { items?: CalendarItemData[] }).items = await loadPool();
+    }
+  }
+}
+
 function isSearchableTextBlock(
   item: unknown,
 ): item is { _type?: string; textFragments?: unknown[] } {