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/ui/Skeleton.tsx

@@ -0,0 +1,44 @@
+import React from 'react';
+import clsx from 'clsx';
+
+export interface SkeletonProps {
+  className?: string;
+  variant?: 'text' | 'rect' | 'circle';
+  width?: string;
+  height?: string;
+}
+
+export function Skeleton({ className, variant = 'rect', width, height }: SkeletonProps) {
+  const variantClass = {
+    text: 'rounded',
+    rect: 'rounded-md',
+    circle: 'rounded-full',
+  };
+
+  return (
+    <div
+      className={clsx(
+        'animate-pulse motion-reduce:animate-none bg-secondary-200',
+        variantClass[variant],
+        className
+      )}
+      style={{ width, height }}
+      role="status"
+      aria-label="Wird geladen"
+    />
+  );
+}
+
+export function SkeletonText({ lines = 3, className }: { lines?: number; className?: string }) {
+  return (
+    <div className={clsx('space-y-2', className)} role="status" aria-label="Wird geladen">
+      {Array.from({ length: lines }).map((_, i) => (
+        <Skeleton
+          key={i}
+          variant="text"
+          className={clsx('h-4', i === lines - 1 && 'w-2/3')}
+        />
+      ))}
+    </div>
+  );
+}