Phase 6.6: Mail-Settings-Forms on RHF + Zod

- MailSettings account form: RHF+Zod (email valid, password required, imap/smtp host required, ports numeric)
- SignatureManager: RHF+Zod (name required, body_html, is_default)
- RuleEditor: RHF+Zod (name required, priority numeric)
- LabelManager: RHF+Zod (name required, color optional)
- VacationResponder: RHF+Zod (enabled, dates with end>start validation, subject, body)
- Error display under each field
- Preserved all existing functionality: CRUD, templates, variables
- Added 2 validation tests for MailSettings account form

frontend/src/__tests__/mail/MailSettings.validation.test.tsx

@@ -0,0 +1,77 @@
+import React from 'react';
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { render, screen, fireEvent, waitFor } from '@testing-library/react';
+import { MemoryRouter } from 'react-router-dom';
+
+vi.mock('@/api/mail', () => ({
+  fetchAccounts: vi.fn().mockResolvedValue([]),
+  createAccount: vi.fn().mockResolvedValue({ id: 'a1', email: 'test@example.com', display_name: '', imap_host: '', imap_port: 993, smtp_host: '', smtp_port: 587, is_shared: false, is_active: true }),
+  deleteAccount: vi.fn(),
+  testConnection: vi.fn(),
+  triggerSync: vi.fn(),
+  updateAccount: vi.fn(),
+  fetchFolders: vi.fn().mockResolvedValue([]),
+}));
+
+vi.mock('@/components/ui/Toast', () => ({
+  useToast: () => ({ success: vi.fn(), error: vi.fn(), info: vi.fn(), warning: vi.fn() }),
+}));
+
+vi.mock('@/components/mail/SignatureManager', () => ({ SignatureManager: () => null }));
+vi.mock('@/components/mail/RuleEditor', () => ({ RuleEditor: () => null }));
+vi.mock('@/components/mail/LabelManager', () => ({ LabelManager: () => null }));
+vi.mock('@/components/mail/VacationResponder', () => ({ VacationResponder: () => null }));
+vi.mock('@/components/mail/PgpSettings', () => ({ PgpSettings: () => null }));
+
+import { MailSettingsPage } from '@/pages/MailSettings';
+import { createAccount } from '@/api/mail';
+
+beforeEach(() => {
+  vi.clearAllMocks();
+});
+
+describe('MailSettings account form validation (RHF + Zod)', () => {
+  it('shows validation error when email is empty', async () => {
+    render(<MemoryRouter><MailSettingsPage /></MemoryRouter>);
+    await waitFor(() => {
+      expect(screen.getByTestId('mail-settings-page')).toBeInTheDocument();
+    }, { timeout: 3000 });
+    // Click add account button
+    const addBtn = screen.getByTestId('add-account-btn');
+    fireEvent.click(addBtn);
+    await waitFor(() => {
+      expect(screen.getByTestId('add-account-form')).toBeInTheDocument();
+    });
+    // Submit empty form
+    const form = screen.getByTestId('add-account-form').querySelector('form');
+    expect(form).toBeTruthy();
+    fireEvent.submit(form!);
+    await waitFor(() => {
+      const errors = screen.getAllByText(/erforderlich|required/i);
+      expect(errors.length).toBeGreaterThan(0);
+    });
+  });
+
+  it('calls createAccount when form is valid', async () => {
+    render(<MemoryRouter><MailSettingsPage /></MemoryRouter>);
+    await waitFor(() => {
+      expect(screen.getByTestId('mail-settings-page')).toBeInTheDocument();
+    }, { timeout: 3000 });
+    const addBtn = screen.getByTestId('add-account-btn');
+    fireEvent.click(addBtn);
+    await waitFor(() => {
+      expect(screen.getByTestId('add-account-form')).toBeInTheDocument();
+    });
+    const form = screen.getByTestId('add-account-form').querySelector('form');
+    const inputs = form!.querySelectorAll('input');
+    // Fill email, imap_host, smtp_host, password
+    fireEvent.change(inputs[0], { target: { value: 'test@example.com' } }); // email
+    fireEvent.change(inputs[3], { target: { value: 'imap.example.com' } }); // imap_host
+    fireEvent.change(inputs[5], { target: { value: 'smtp.example.com' } }); // smtp_host
+    fireEvent.change(inputs[7], { target: { value: 'password123' } }); // password
+    fireEvent.submit(form!);
+    await waitFor(() => {
+      expect(createAccount).toHaveBeenCalledWith(expect.objectContaining({ email: 'test@example.com', imap_host: 'imap.example.com', smtp_host: 'smtp.example.com' }));
+    }, { timeout: 3000 });
+  });
+});