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/+page.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+</script>
+
+<div class="flex flex-1 items-center justify-center p-8">
+	<p class="text-sm text-ink-3">{m.admin_groups_select_prompt()}</p>
+</div>