Phase 6.3: SettingsForms on RHF + Zod

- SettingsCurrencies: RHF+Zod (code required max 3, name required, symbol required max 5)
- SettingsTaxes: RHF+Zod (name required, rate numeric 0-100, country max 2)
- SettingsSequences: RHF+Zod (name required, padding numeric 1-10)
- SettingsUsers: RHF+Zod (name required, email valid, password min 8)
- SettingsRoles: RHF+Zod for create form (name required)
- SettingsGroups: RHF+Zod for create form (name required, description optional)
- Error display under each field
- Preserved all existing functionality: CRUD, permissions, members
- Added 2 validation tests for SettingsCurrencies

frontend/src/__tests__/settings/SettingsCurrencies.validation.test.tsx

@@ -0,0 +1,55 @@
+import React from 'react';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+
+vi.mock('@/api/client', () => ({
+  apiGet: vi.fn().mockResolvedValue({ items: [], total: 0 }),
+  apiPost: vi.fn().mockResolvedValue({ id: '1', code: 'EUR', name: 'Euro', symbol: '€', is_default: true }),
+  apiPatch: vi.fn().mockResolvedValue({}),
+  apiDelete: vi.fn().mockResolvedValue({}),
+}));
+
+import { SettingsCurrenciesPage } from '@/pages/SettingsCurrencies';
+import { apiPost } from '@/api/client';
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe('SettingsCurrencies validation (RHF + Zod)', () => {
+  it('shows validation error when code is empty', async () => {
+    render(<SettingsCurrenciesPage />);
+    await waitFor(() => {
+      expect(screen.getByTestId('settings-currencies')).toBeInTheDocument();
+    });
+    // Click add button to show form (German locale default in test env)
+    const addBtn = screen.getByText(/währung hinzufügen|add currency/i);
+    fireEvent.click(addBtn);
+    // Submit empty form
+    const saveBtn = screen.getByRole('button', { name: /speichern|^save$/i });
+    fireEvent.click(saveBtn);
+    await waitFor(() => {
+      const errors = screen.getAllByText(/erforderlich|required/i);
+      expect(errors.length).toBeGreaterThan(0);
+    });
+  });
+
+  it('submits successfully with valid data', async () => {
+    render(<SettingsCurrenciesPage />);
+    await waitFor(() => {
+      expect(screen.getByTestId('settings-currencies')).toBeInTheDocument();
+    });
+    const addBtn = screen.getByText(/währung hinzufügen|add currency/i);
+    fireEvent.click(addBtn);
+    // Fill form — code, symbol, name
+    const inputs = screen.getAllByRole('textbox');
+    fireEvent.change(inputs[0], { target: { value: 'EUR' } }); // code
+    fireEvent.change(inputs[1], { target: { value: '€' } }); // symbol
+    fireEvent.change(inputs[2], { target: { value: 'Euro' } }); // name
+    const saveBtn = screen.getByRole('button', { name: /speichern|^save$/i });
+    fireEvent.click(saveBtn);
+    await waitFor(() => {
+      expect(apiPost).toHaveBeenCalledWith('/currencies', expect.objectContaining({ code: 'EUR', name: 'Euro', symbol: '€' }));
+    });
+  });
+});