refactor: move user domain components to lib/user/

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

frontend/src/lib/user/UserGroupsSection.svelte

@@ -0,0 +1,26 @@
+<script lang="ts">
+let {
+	groups,
+	selectedGroupIds = []
+}: {
+	groups: { id: string; name: string }[];
+	selectedGroupIds?: string[];
+} = $props();
+
+let selected = $derived([...selectedGroupIds]);
+</script>
+
+<div class="flex flex-wrap gap-3">
+	{#each groups as group (group.id)}
+		<label class="inline-flex items-center gap-2 text-sm text-ink-2">
+			<input
+				type="checkbox"
+				name="groupIds"
+				value={group.id}
+				bind:group={selected}
+				class="rounded border-line text-ink focus:ring-focus-ring"
+			/>
+			{group.name}
+		</label>
+	{/each}
+</div>