feat(C): Phase C — Core UI prüfen, vervollständigen, testen

C-ERR-BOUNDARY: ErrorBoundary erweitert (trace_id, Fallback-UI, PluginErrorBoundary)
C-NOTIF: NotificationDropdown mit System-Channel-Link
C-DOCS: ApiDocs.tsx Seite (Swagger UI iframe)
C-A11Y: aria-label, min-h-touch, focus-visible ergänzt
C-FE-TEST: 29 neue Tests (ErrorBoundary, ApiDocs, PrintButton, themeStore, NotificationDropdown)
C-DOC: ui-design-guidelines.md aktualisiert

Verifiziert (keine Änderungen nötig):
- C-TAGS, C-CF, C-FILTER, C-DEDUP, C-ONBOARD, C-THEME, C-PWA, C-PRINT

TSC: 0 errors, Build: erfolgreich, Vitest: 29/29 passed

frontend/src/__tests__/pages/ApiDocs.test.tsx

@@ -0,0 +1,36 @@
+/**
+ * ApiDocs page tests — verifies iframe embedding and external link.
+ */
+
+import React from 'react';
+import { describe, it, expect, vi } from 'vitest';
+import { render, screen } from '@testing-library/react';
+import { ApiDocsPage } from '@/pages/ApiDocs';
+
+// Mock react-i18next
+vi.mock('react-i18next', () => ({
+  useTranslation: () => ({ t: (key: string, fallback: string) => fallback }),
+}));
+
+describe('ApiDocsPage', () => {
+  it('renders the page with title', () => {
+    render(<ApiDocsPage />);
+    expect(screen.getByTestId('api-docs-page')).toBeInTheDocument();
+    expect(screen.getByText('API-Dokumentation')).toBeInTheDocument();
+  });
+
+  it('embeds an iframe pointing to /docs', () => {
+    render(<ApiDocsPage />);
+    const iframe = screen.getByTitle('API-Dokumentation');
+    expect(iframe.tagName).toBe('IFRAME');
+    expect(iframe).toHaveAttribute('src', '/docs');
+  });
+
+  it('has a link to open /docs in a new tab', () => {
+    render(<ApiDocsPage />);
+    const link = screen.getByRole('link', { name: 'In neuem Tab öffnen' });
+    expect(link).toHaveAttribute('href', '/docs');
+    expect(link).toHaveAttribute('target', '_blank');
+    expect(link).toHaveAttribute('rel', 'noopener noreferrer');
+  });
+});