fix(ui): make Button.children optional in props type

svelte-check (strict) flagged ContactFormComponent for not passing
children to <Button>. The component already renders label as fallback;
typed children as optional Snippet so callers don't need to pass it.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/ui/Button.svelte

@@ -2,6 +2,8 @@
   type Variant = 'primary' | 'secondary' | 'ghost';
   type Size = 'sm' | 'md' | 'large';
 
+  import type { Snippet } from 'svelte';
+
   let {
     variant = 'primary',
     size = 'md',
@@ -9,7 +11,15 @@
     loading = false,
     type = 'button' as 'button' | 'submit' | 'reset',
     label = '',
-    children,
+    children = undefined,
+  }: {
+    variant?: 'primary' | 'secondary' | 'ghost';
+    size?: 'sm' | 'md' | 'large';
+    disabled?: boolean;
+    loading?: boolean;
+    type?: 'button' | 'submit' | 'reset';
+    label?: string;
+    children?: Snippet;
   } = $props();
 
   const base =