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.server.ts

@@ -0,0 +1,8 @@
+import { createApiClient } from '$lib/api.server';
+import type { LayoutServerLoad } from './$types';
+
+export const load: LayoutServerLoad = async ({ fetch }) => {
+	const api = createApiClient(fetch);
+	const result = await api.GET('/api/groups');
+	return { groups: result.data ?? [] };
+};