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/Card.svelte

@@ -7,7 +7,7 @@
    * tile + post: PostCard – Shell ohne Padding, Medien bündig, Text mit p-4 im Kind.
    * callout: CTA; layout wird ignoriert.
    */
-  const cardSurface = cva("relative min-w-0 overflow-hidden", {
+  const cardSurface = cva("relative min-w-0 overflow-hidden rounded-sm", {
     variants: {
       variant: {
         tile: "",
@@ -24,20 +24,17 @@
       {
         variant: "tile",
         layout: "stack",
-        class:
-          "flex flex-col gap-2 border border-stein-200 bg-white p-4 text-stein-900",
+        class: "card-surface flex flex-col gap-2 p-4 text-stein-900",
       },
       {
         variant: "tile",
         layout: "inline",
-        class:
-          "flex flex-row items-start gap-3 border border-stein-200 bg-white p-4 text-stein-900",
+        class: "card-surface flex flex-row items-start gap-3 p-4 text-stein-900",
       },
       {
         variant: "tile",
         layout: "post",
-        class:
-          "flex flex-col gap-0 border border-stein-200 bg-white p-0 text-stein-900",
+        class: "card-surface flex flex-col gap-0 p-0 text-stein-900",
       },
     ],
     defaultVariants: { variant: "tile", layout: "stack" },