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/new/+page.server.ts

@@ -0,0 +1,25 @@
+import { fail, redirect } from '@sveltejs/kit';
+import type { Actions } from './$types';
+import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
+
+export const actions: Actions = {
+	default: async ({ request, fetch }) => {
+		const data = await request.formData();
+		const api = createApiClient(fetch);
+
+		const result = await api.POST('/api/groups', {
+			body: {
+				name: data.get('name') as string,
+				permissions: data.getAll('permissions') as string[]
+			}
+		});
+
+		if (!result.response.ok) {
+			const code = (result.error as unknown as { code?: string })?.code;
+			return fail(result.response.status, { error: getErrorMessage(code) });
+		}
+
+		throw redirect(303, '/admin/groups');
+	}
+};