fix(invites): make group checkboxes writable — $derived → $state

bind:group requires a writable $state variable; $derived is read-only
in Svelte 5, so every click was silently reset to unchecked, making
the group picker non-functional.

Also wraps checkboxes in <fieldset>/<legend> for WCAG 1.3.1 compliance.

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

frontend/src/lib/user/UserGroupsSection.svelte

@@ -7,10 +7,11 @@ let {
 	selectedGroupIds?: string[];
 } = $props();
 
-let selected = $derived([...selectedGroupIds]);
+let selected = $state<string[]>([...selectedGroupIds]);
 </script>
 
-<div class="flex flex-wrap gap-3">
+<fieldset class="flex flex-wrap gap-3 border-none p-0">
+	<legend class="sr-only">Gruppen</legend>
 	{#each groups as group (group.id)}
 		<label class="inline-flex items-center gap-2 text-sm text-ink-2">
 			<input
@@ -23,4 +24,4 @@ let selected = $derived([...selectedGroupIds]);
 			{group.name}
 		</label>
 	{/each}
-</div>
+</fieldset>