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/components/layout/AppShell.tsx

@@ -0,0 +1,26 @@
+import React from 'react';
+import { Outlet } from 'react-router-dom';
+import { Sidebar } from './Sidebar';
+import { TopBar } from './TopBar';
+import { ToastContainer } from '@/components/ui/Toast';
+
+export function AppShell() {
+  return (
+    <div className="flex h-screen overflow-hidden bg-secondary-50" data-testid="app-shell">
+      <Sidebar />
+      <div className="flex-1 flex flex-col overflow-hidden">
+        <TopBar />
+        <main
+          className="flex-1 overflow-y-auto focus:outline-none"
+          tabIndex={-1}
+          role="main"
+          id="main-content"
+          data-testid="content-area"
+        >
+          <Outlet />
+        </main>
+      </div>
+      <ToastContainer />
+    </div>
+  );
+}