ui: card-surface utility, rounded corners, calendar polish, banner layout

- Add .card-surface utility (@layer components): rounded-sm border shadow-sm
- Card.svelte, CalendarBlock, SearchableTextBlock, BlogOverview use card-surface
- OrganisationsBlock: logo h-12 rounded-sm, description style cleanup, pt-1 for hover clip fix
- DeadlineBanner: container-custom width constraint, text left-aligned
- WindAreaPanel stats: card-surface
- CalendarBlock: rounded-sm day headers, past days opacity-20 + diagonal strike
- Add .day-past pseudo-element utility for diagonal line

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

src/lib/components/BlogOverview.svelte

@@ -235,7 +235,7 @@
                 {#each filteredResults as hit}
                     <a
                         href={searchResultHref(hit.slug)}
-                        class="flex flex-col overflow-hidden border border-stein-200 bg-white text-stein-900 no-underline transition-colors"
+                        class="card-surface flex flex-col overflow-hidden text-stein-900 no-underline transition-colors hover:-translate-y-0.5 hover:shadow-md hover:border-wald-300 transition-[transform,box-shadow,border-color] duration-200"
                     >
                         {#if hit.image}
                             <div