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/CalendarBlock.svelte

@@ -536,7 +536,7 @@
 </script>
 
 <div
-    class="{layoutClasses} w-full min-w-0 calendar-block border border-stein-200 overflow-hidden"
+    class="{layoutClasses} w-full min-w-0 calendar-block card-surface overflow-hidden"
     data-block="Calendar"
     data-block-type="calendar"
     data-block-slug={block._slug}
@@ -1044,7 +1044,7 @@
             <div class="grid grid-cols-7 gap-1.5 text-center text-sm">
                 {#each weekdays as w, i}
                     <div
-                        class="py-1.5 text-xs font-bold rounded-xs {i >= 5
+                        class="py-1.5 text-xs font-bold rounded-sm {i >= 5
                             ? 'bg-stein-100/80 text-stein-600'
                             : 'bg-himmel-200/50 text-himmel-800'}"
                     >
@@ -1068,6 +1068,7 @@
                             <button
                                 type="button"
                                 class="relative h-10 rounded-xs flex flex-col items-center justify-center min-w-0 transition-colors cursor-pointer font-semibold
+                                    {!isFuture ? 'opacity-20 day-past' : ''}
                                     {isCurrentMonth
                                     ? 'text-himmel-900'
                                     : 'text-himmel-700 opacity-60 hover:opacity-80'}
@@ -1106,6 +1107,7 @@
                         {:else}
                             <div
                                 class="relative h-10 rounded-xs flex flex-col items-center justify-center min-w-0 cursor-default
+                                    {!isFuture ? 'opacity-20 day-past' : ''}
                                     {isWeekend
                                     ? 'bg-stein-100/60'
                                     : 'bg-himmel-50/50'}