Restructures frontend to a feature-based folder layout

Groups auth, setup, invitation, profile, users, cms, availability and
system code (services/hooks, components, schemas, mocks, pages) under
src/features/<name> instead of splitting by technical layer (api/,
components/, lib/schemas/, mocks/, pages/). Renames the old
connection-oriented `api` layer to `services` per feature, and splits
the monolithic api/types.ts into per-feature types.ts files (with
ProblemDetails/ApiResult merged into lib/api-client.ts as shared
infra). Layout-agnostic code (ui primitives, app shell, i18n, test
utils, lib) stays at the top level.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/cms/services/useAddCmsInstance.test.ts

@@ -0,0 +1,54 @@
+import { renderHook, waitFor, act } from '@testing-library/react';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { http, HttpResponse } from 'msw';
+import { createElement } from 'react';
+import { describe, expect, it, vi } from 'vitest';
+import { server } from '@/mocks/server';
+import { API_BASE } from '@/features/auth/mocks/fixtures';
+import { useAddCmsInstance } from './useAddCmsInstance';
+
+function createWrapper() {
+    const queryClient = new QueryClient({
+        defaultOptions: { queries: { retry: false }, mutations: { retry: false } },
+    });
+    return ({ children }: { children: React.ReactNode }) =>
+        createElement(QueryClientProvider, { client: queryClient }, children);
+}
+
+const CMS_URL = `${API_BASE}/api/v1/CmsInstances`;
+
+describe('useAddCmsInstance', () => {
+    it('returns created instance on success', async () => {
+        const { result } = renderHook(() => useAddCmsInstance(), { wrapper: createWrapper() });
+
+        await act(async () => {
+            await result.current.mutateAsync({
+                name: 'Test CMS',
+                url: 'http://test.example.com',
+                apiKey: 'secret',
+            });
+        });
+
+        await waitFor(() => expect(result.current.isSuccess).toBe(true));
+        expect(result.current.data?.name).toBe('Test CMS');
+    });
+
+    it('calls onError callback on HTTP 400', async () => {
+        server.use(http.post(CMS_URL, () => HttpResponse.json({}, { status: 400 })));
+
+        const onError = vi.fn();
+        const { result } = renderHook(() => useAddCmsInstance({ onError }), {
+            wrapper: createWrapper(),
+        });
+
+        await act(async () => {
+            try {
+                await result.current.mutateAsync({ name: 'X', url: 'http://x.com', apiKey: 'k' });
+            } catch {
+                // expected
+            }
+        });
+
+        await waitFor(() => expect(onError).toHaveBeenCalled());
+    });
+});