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/main.tsx

@@ -4,8 +4,8 @@ import { RouterProvider } from '@tanstack/react-router';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import './index.css';
 import './i18n/config';
-import { AuthProvider } from '@/contexts/AuthProvider';
-import { useAuth } from '@/contexts/auth-context';
+import { AuthProvider } from '@/features/auth/context/AuthProvider';
+import { useAuth } from '@/features/auth/context/auth-context';
 import { Toaster } from '@/components/ui/sonner';
 import { router } from '@/router';
 import { getAppConfig } from '@/lib/config';