feat(admin/groups): add groups entity with master-detail sub-routes

Creates the full groups section under /admin/groups/:
- +layout.server.ts: loads groups list via GET /api/groups
- GroupsListPanel.svelte: left list panel (name + permission count, active state)
- +layout.svelte: composes list panel + children slot
- +page.svelte: empty selection prompt
- [id]/+page.server.ts: update (PATCH) and delete actions
- [id]/+page.svelte: edit detail panel with Standard/Administrative permission sections
- new/+page.svelte and +page.server.ts: create group form

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

frontend/src/routes/admin/groups/GroupsListPanel.svelte

@@ -0,0 +1,64 @@
+<script lang="ts">
+import { page } from '$app/state';
+import { m } from '$lib/paraglide/messages.js';
+
+type Group = {
+	id: string;
+	name: string;
+	permissions: string[];
+};
+
+let { groups }: { groups: Group[] } = $props();
+</script>
+
+<div class="flex w-60 flex-shrink-0 flex-col overflow-hidden border-r border-line bg-surface">
+	<!-- Panel header -->
+	<div class="flex items-center justify-between border-b border-line px-3 py-2">
+		<span class="text-xs font-bold tracking-widest text-ink-3 uppercase">
+			{m.admin_groups_list_title()}
+		</span>
+		<a
+			href="/admin/groups/new"
+			class="inline-flex items-center gap-1 rounded-sm px-2 py-1 text-xs font-medium text-ink-2 transition-colors hover:bg-muted hover:text-ink"
+			title={m.admin_btn_new_group()}
+			aria-label={m.admin_btn_new_group()}
+		>
+			<svg
+				class="h-3.5 w-3.5"
+				fill="none"
+				viewBox="0 0 24 24"
+				stroke="currentColor"
+				stroke-width="2.5"
+				aria-hidden="true"
+			>
+				<path stroke-linecap="round" stroke-linejoin="round" d="M12 4v16m8-8H4" />
+			</svg>
+			{m.admin_btn_new_group()}
+		</a>
+	</div>
+
+	<!-- Scrollable group list -->
+	<div class="flex-1 overflow-y-auto">
+		{#if groups.length === 0}
+			<p class="px-4 py-6 text-center text-xs text-ink-3">
+				{m.admin_groups_empty()}
+			</p>
+		{:else}
+			{#each groups as group (group.id)}
+				{@const isActive = page.url.pathname.startsWith('/admin/groups/' + group.id)}
+				<a
+					href="/admin/groups/{group.id}"
+					aria-current={isActive ? 'page' : undefined}
+					class="block border-l-2 px-3 py-2.5 transition-colors {isActive
+						? 'border-primary bg-primary/10 dark:bg-primary/15'
+						: 'border-transparent hover:bg-muted'}"
+				>
+					<div class="text-sm font-bold text-ink">{group.name}</div>
+					<div class="mt-0.5 text-xs text-ink-3">
+						{m.admin_groups_permission_count({ count: group.permissions.length })}
+					</div>
+				</a>
+			{/each}
+		{/if}
+	</div>
+</div>