T07a: frontend core SPA — shell + auth + routing + i18n + UI library + a11y

- React 18 + Vite + TypeScript + Tailwind CSS setup
- AppShell with Sidebar (plugin menu) + TopBar (tenant switcher, search, notifications, user menu)
- Auth pages: Login, PasswordResetRequest, PasswordResetConfirm
- Protected routes with auth guard
- API client (axios with interceptors: session cookie, 401 redirect, 422 validation)
- TanStack Query hooks for auth, users, companies, contacts, notifications
- Zustand stores: authStore, uiStore
- i18n setup (de/en locales) with react-i18next
- UI component library: Button, Input, Select, Modal, Toast, Table, Card, Badge, Avatar, Pagination, EmptyState, Skeleton, ConfirmDialog
- Accessibility: ARIA labels, 44px touch targets, keyboard nav, reduced-motion, sr-only
- Design tokens from prototype as CSS custom properties
- 111 tests passing across 20 test files
- tsc --noEmit: 0 errors
- npm run build: success (471KB JS, 24KB CSS)

frontend/src/store/authStore.ts

@@ -0,0 +1,55 @@
+import { create } from 'zustand';
+
+export interface Tenant {
+  id: string;
+  name: string;
+  slug: string;
+}
+
+export interface User {
+  id: string;
+  email: string;
+  first_name: string;
+  last_name: string;
+  role: string;
+  avatar_url: string | null;
+  tenants: Tenant[];
+}
+
+export interface AuthState {
+  user: User | null;
+  currentTenant: Tenant | null; isAuthenticated: boolean;
+  isLoading: boolean;
+  error: string | null;
+  setUser: (user: User | null) => void;
+  setTenant: (tenant: Tenant | null) => void;
+  setAuthenticated: (authed: boolean) => void;
+  setLoading: (loading: boolean) => void;
+  setError: (error: string | null) => void;
+  logout: () => void;
+}
+
+export const useAuthStore = create<AuthState>((set) => ({
+  user: null,
+  currentTenant: null,
+  isAuthenticated: false,
+  isLoading: false,
+  error: null,
+  setUser: (user) =>
+    set({
+      user,
+      isAuthenticated: !!user,
+      currentTenant: user?.tenants?.[0] ?? null,
+    }),
+  setTenant: (tenant) => set({ currentTenant: tenant }),
+  setAuthenticated: (authed) => set({ isAuthenticated: authed }),
+  setLoading: (loading) => set({ isLoading: loading }),
+  setError: (error) => set({ error }),
+  logout: () =>
+    set({
+      user: null,
+      currentTenant: null,
+      isAuthenticated: false,
+      error: null,
+    }),
+}));