feat(admin): responsive entity nav and collapsible list panels (Phase 9)

EntityNav: hidden on mobile, 48px icon strip at tablet (md), full labels+counts at desktop (lg).
Each list panel collapses to a 32px handle via localStorage-persisted state; auto-collapses when
navigating to the "+New" route. Mobile routing hides the list panel when a detail route is active.

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

frontend/src/routes/admin/groups/+layout.svelte

@@ -1,12 +1,17 @@
 <script lang="ts">
+import { page } from '$app/state';
 import GroupsListPanel from './GroupsListPanel.svelte';
 
 let { data, children } = $props();
+
+const autoCollapse = $derived(page.url.pathname === '/admin/groups/new');
+const isAtListRoot = $derived(page.url.pathname === '/admin/groups');
 </script>
 
-<GroupsListPanel groups={data.groups} />
+<div class="{isAtListRoot ? 'flex' : 'hidden'} flex-shrink-0 md:flex">
+	<GroupsListPanel groups={data.groups} autocollapse={autoCollapse} />
+</div>
 
-<!-- Detail panel -->
-<div class="flex min-w-0 flex-1 flex-col overflow-hidden">
+<div class="{isAtListRoot ? 'hidden' : 'flex'} min-w-0 flex-1 flex-col overflow-hidden md:flex">
 	{@render children()}
 </div>