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/blocks/DeadlineBannerBlock.svelte

@@ -116,10 +116,10 @@
         data-block-slug={block._slug}
     >
         <div
-            class="gradient-blob-bg flex items-center gap-x-6 bg-gray-50 px-6 py-2.5 sm:px-3.5"
+            class="gradient-blob-bg bg-gray-50 py-2.5"
         >
-            <span class="hidden flex-1 sm:block" aria-hidden="true"></span>
-            <p class="my-0! text-sm/6 text-gray-900">
+            <div class="container-custom flex items-center gap-x-6">
+            <p class="my-0! flex-1 text-sm/6 text-gray-900">
                 {sentence}{#if countdownStr}{sentence ? " · " : ""}<span
                         class="font-semibold">{countdownStr}</span
                     >{/if}{#if href}
@@ -137,7 +137,7 @@
                     </a>
                 {/if}
             </p>
-            <div class="flex flex-1 justify-end">
+            <div class="shrink-0">
                 <button
                     type="button"
                     onclick={() => (dismissed = true)}
@@ -151,6 +151,7 @@
                     />
                 </button>
             </div>
+            </div>
         </div>
     </div>
 {/if}