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/index.css

@@ -0,0 +1,93 @@
+@tailwind base;
+@tailwind components;
+@tailwind utilities;
+
+:root {
+  /* Design tokens from prototype */
+  --color-primary: #2563eb;
+  --color-primary-light: #3b82f6;
+  --color-primary-dark: #1d4ed8;
+  --color-secondary: #64748b;
+  --color-accent: #d946ef;
+  --color-danger: #dc2626;
+  --color-warning: #f59e0b;
+  --color-success: #16a34a;
+  --color-bg: #ffffff;
+  --color-bg-secondary: #f8fafc;
+  --color-bg-tertiary: #f1f5f9;
+  --color-border: #e2e8f0;
+  --color-text: #0f172a;
+  --color-text-muted: #64748b;
+  --color-text-light: #94a3b8;
+  --radius-sm: 0.375rem;
+  --radius-md: 0.5rem;
+  --radius-lg: 0.75rem;
+  --shadow-sm: 0 1px 2px 0 rgba(0,0,0,0.05);
+  --shadow-md: 0 4px 6px -1px rgba(0,0,0,0.1), 0 2px 4px -2px rgba(0,0,0,0.1);
+  --shadow-lg: 0 10px 15px -3px rgba(0,0,0,0.1), 0 4px 6px -4px rgba(0,0,0,0.1);
+  --font-sans: 'Inter', system-ui, -apple-system, sans-serif;
+  --font-mono: 'JetBrains Mono', monospace;
+  --touch-target: 44px;
+}
+
+.dark {
+  --color-bg: #0f172a;
+  --color-bg-secondary: #1e293b;
+  --color-bg-tertiary: #334155;
+  --color-border: #334155;
+  --color-text: #f1f5f9;
+  --color-text-muted: #94a3b8;
+  --color-text-light: #64748b;
+}
+
+@layer base {
+  * {
+    @apply border-secondary-200;
+  }
+  body {
+    @apply bg-secondary-50 text-secondary-900 font-sans antialiased;
+    margin: 0;
+    min-height: 100vh;
+  }
+  #root {
+    min-height: 100vh;
+  }
+}
+
+@layer components {
+  .btn-touch {
+    @apply min-h-touch min-w-touch;
+  }
+  .focus-ring {
+    @apply focus:outline-none focus-visible:ring-2 focus-visible:ring-primary-500 focus-visible:ring-offset-2;
+  }
+  .sr-only-focusable:not(:focus):not(:focus-within) {
+    @apply sr-only;
+  }
+}
+
+/* Reduced motion support */
+@media (prefers-reduced-motion: reduce) {
+  *, *::before, *::after {
+    animation-duration: 0.01ms !important;
+    animation-iteration-count: 1 !important;
+    transition-duration: 0.01ms !important;
+    scroll-behavior: auto !important;
+  }
+}
+
+/* Custom scrollbar */
+::-webkit-scrollbar {
+  width: 8px;
+  height: 8px;
+}
+::-webkit-scrollbar-track {
+  background: transparent;
+}
+::-webkit-scrollbar-thumb {
+  background: #cbd5e1;
+  border-radius: 4px;
+}
+::-webkit-scrollbar-thumb:hover {
+  background: #94a3b8;
+}