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/+layout.svelte

@@ -0,0 +1,12 @@
+<script lang="ts">
+import GroupsListPanel from './GroupsListPanel.svelte';
+
+let { data, children } = $props();
+</script>
+
+<GroupsListPanel groups={data.groups} />
+
+<!-- Detail panel -->
+<div class="flex min-w-0 flex-1 flex-col overflow-hidden">
+	{@render children()}
+</div>