refactor(admin/users): extract user form sub-components

Shared (src/lib/components/user/):
- UserProfileSection.svelte: name/birth-date/email/contact fields
- UserGroupsSection.svelte: group checkboxes
- UserPasswordSection.svelte: new/confirm password fields

Page-local:
- admin/users/new/AccountSection.svelte: username + initial password

admin/users/[id] drops from 224 → ~35 lines.
admin/users/new drops from 191 → ~30 lines.
Date utilities imported from \$lib/utils/date.

Part of #75

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

frontend/src/lib/components/user/UserGroupsSection.svelte

@@ -0,0 +1,24 @@
+<script lang="ts">
+let {
+	groups,
+	selectedGroupIds = []
+}: {
+	groups: { id: string; name: string }[];
+	selectedGroupIds?: string[];
+} = $props();
+</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}
+				checked={selectedGroupIds.includes(group.id)}
+				class="rounded border-line text-ink focus:ring-accent"
+			/>
+			{group.name}
+		</label>
+	{/each}
+</div>