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/auth/context/auth-context.ts

@@ -0,0 +1,27 @@
+import { createContext, useContext } from 'react';
+import type { User } from '@/features/auth/services/types';
+
+export type AuthStatus = 'loading' | 'authenticated' | 'guest';
+
+export interface AuthContextValue {
+    status: AuthStatus;
+    user: User | null;
+    /** Access token, kept only in memory (BR-U1-02). */
+    accessToken: string | null;
+    expiresAt: string | null;
+    isAuthenticated: boolean;
+    login: (email: string, password: string) => Promise<void>;
+    logout: () => Promise<void>;
+    /** Performs a cookie-based silent refresh; resolves to the new token or null. */
+    refresh: () => Promise<string | null>;
+}
+
+export const AuthContext = createContext<AuthContextValue | null>(null);
+
+export function useAuth(): AuthContextValue {
+    const ctx = useContext(AuthContext);
+    if (ctx === null) {
+        throw new Error('useAuth must be used within an AuthProvider');
+    }
+    return ctx;
+}