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/app.css

@@ -10,6 +10,32 @@
 
 @import "tailwindcss";
 
+/* ==========================================================================
+   Component Utilities
+   ========================================================================== */
+
+@layer components {
+  /* Shared card surface: border, background, shadow, radius */
+  .card-surface {
+    @apply rounded-sm border border-stein-200 bg-white shadow-sm;
+  }
+
+  /* Diagonal strikethrough for past calendar days */
+  .day-past::after {
+    content: '';
+    position: absolute;
+    inset: 2px;
+    background: linear-gradient(
+      to top right,
+      transparent calc(50% - 0.5px),
+      rgba(0, 0, 0, 0.12) calc(50% - 0.5px),
+      rgba(0, 0, 0, 0.12) calc(50% + 0.5px),
+      transparent calc(50% + 0.5px)
+    );
+    pointer-events: none;
+  }
+}
+
 /* ==========================================================================
    Design Tokens – Windwiderstand (01-colors, 02-typography, 06-tokens.json)
    ========================================================================== */