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/routes/[...slug]/+page.server.ts

@@ -10,6 +10,7 @@ import {
   resolvePostOverviewBlocks,
   resolveSearchableTextBlocks,
   resolveCalendarBlocks,
+  resolveDeadlineBannerBlocks,
 } from '$lib/blog-utils';
 import { PAGE_RESOLVE } from '$lib/constants';
 import { error } from '@sveltejs/kit';
@@ -39,6 +40,7 @@ export const load: PageServerLoad = async ({ params, locals, fetch }) => {
   const tagsMap = await resolvePostOverviewBlocks(page);
   await resolveSearchableTextBlocks(page, tagsMap);
   await resolveCalendarBlocks(page);
+  await resolveDeadlineBannerBlocks(page);
 
   // Resolve top banner if present
   let topBannerResolvedImages: string[] = [];