feat(calendar): CalendarCompact nutzt SectionHeader; fix IconButton-Typing

CalendarCompactBlock: plain h3 → SectionHeader (title + 'Alle kommenden Termine'-
Action). IconButton props via HTMLButton/AnchorAttributes statt Index-Signatur
(brach Storybook-Typing).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/IconButton.svelte

@@ -6,6 +6,7 @@
    */
   import Icon from "@iconify/svelte";
   import "$lib/iconify-offline";
+  import type { HTMLButtonAttributes, HTMLAnchorAttributes } from "svelte/elements";
 
   type Size = "sm" | "md";
   type Variant = "surface" | "ghost";
@@ -29,8 +30,8 @@
     variant?: Variant;
     disabled?: boolean;
     class?: string;
-    [key: string]: unknown;
-  } = $props();
+  } & HTMLButtonAttributes &
+    HTMLAnchorAttributes = $props();
 
   const sizeClass: Record<Size, string> = { sm: "size-7", md: "size-9" };
   const iconClass: Record<Size, string> = { sm: "size-4", md: "size-5" };